I think your sentence would sound like: Tá na héadaí riachtanacha ann. The noun in your sentence must have a definite article ('na' for plural + h-prothesis) and it has to agree with the adjective (if it is a part of a nominal phrase), so the 'riachtanach' is 'riachtanacha' in plural (because 'éadaí' is plural). If the adjective is part of the predicate, then it doesn't agree with the noun in case or number. Also, I think the English 'there is/are' is 'tá ann' in Irish (with the 'ann' meaning there).
Nikola is partly right. I believe what you're thinking of is this sort of sentence (pulled from a Phrases lesson):
Ta uisce romhat.
The literal translation is "Water is before you" or "Water is in front of you." But the more common way to say it is, "There is water before you," or "There is water in front of you."
This sort of thing came up in a few other lessons as well, with "There is/There are" as the preferred translation.
For "Tá éadaí riachtanach" to be "There is/There are," the best I can come up with is maybe "There are necessary clothes," but "Clothes are necessary" is much more natural. It's best to keep the meaning of a sentence in mind. The literal translation isn't necessarily the correct one.
The thing you are dealing with here is that English is weird! Irish is fairly straight forward in this respect, but is there an obvious reason that in English, with an indefinite article (or no article) like "an apple", you can say "there is an apple in the box" but with a definite article like "the apple", you can't use "there is", you have to say "the apple is in the box"?
So you have
tá úll sa bhosca - "there is an apple in the box" but
tá an t-úll sa bhosca - "the apple is in the box"
That's not an oddity in Irish, it's an oddity in English!
Yes, it is also grammatically possible to translate tá úll sa bhosca as "an apple is in the box". But it is a bit unnatural. You wouldn't answer the question "Is there anything to eat?" with "an apple is in the box", but you could say "there is an apple in the box".
OK. But there isn't a definite article in tá éadaí riachtanach, so why isn't it "There are necessary clothes"? Well, tá éadaí riachtanacha sa bhosca does mean "there are necessary clothes in the box", but you'll notice that riachtanacha is plural because éadaí is plural, and "necessary" is an attributive adjective (attributive adjectives agree with their nouns in case, number and gender). With tá éadaí riachtanach, riachtanach is not plural, so you know that it's a predicative adjective rather than an attributive adjective, and therefore tá éadaí riachtanach means "clothes are necessary" (because a predicative adjective comes after the verb in English). In English, attributive nouns usually go before the noun, and predicative nouns come after, so if you can tell that the adjective is a predicative adjective, you know that you don't use the "there" construction in English.
But you can't always tell whether the adjective is attributive or predicative in Irish (both types normally come afer the noun in Irish) and if your noun is a singular masculine noun, your attributive adjective will agree by not having any changes, so you might have to use your judgement - tá an fear fluich san uisce could be either "the man is wet in the water" (predicative fluich) or "the wet man is in the water" (attributive fluich), but tá an fear mór san uisce is almost certainly "the big man is in the water" and not "the man is big in the water", so mór is much more likely to be attributive in this case. It's not grammatically impossible that it's predicative, just very unlikely, which is why it's a judgement call. Without the definite article, tá fear fluich san uisce is probably "there is a wet man in the water" (attributive) rather than "a man is wet in the water" predicative.
So that's what's involved in deciding whether the English translation of tá uses "there" or not. Look for definite articles, and look at the adjectives.
Note that there is a different "there" in English - if you are pointing at the "necessary clothes", and say "there are the necessary clothes", you would say something like sin iad na héadaí riachtanacha or tá na héadaí riachtanacha ansin - "the necessary clothes are there" - that "there" is indicating position.
By half understanding your explanation I now fully understand why I have to stop trying to learn Irish. Please don't take that as criticism, it genuinely isn't. You've prevented me from wasting anymore of my life on this language and I do appreciate that.
I wasn't taught any grammar at all and I've had no time to learn it since as a discrete discipline. So, without grammar the closest translation of this I could hope for is "Necessary clothing", which is wrong for the reasons which you explained, but which I can never hope to acquire knowledge of.
Children in Irish speaking areas obviously pick up the language without needing the rosetta stone of grammar to decode Irish; but adults trying to learn it very obviously do need to gain a vast knowledge of grammar first to be in with a cat in hell's chance of even getting to first base.
Ah well, it's not to be for me then. I have to be realistic I can never know this language now. I think I sort of realised this years ago at school in Ireland but didn't know why. At least I've got closure with the damn thing now and understand why I can't learn it.
Millions of people have learned to speak Irish without a notion about formal grammar - most of the grammar here is just a set of labels that makes it easier to compare things both within and between languages.
But if you've made up your mind that you can't learn grammar, you just have to take a different approach to learning. Duolingo was actually designed to teach languages without teaching formal grammar, by spaced repetition, allowing you to figure out the difference between "clothes are necessary" and "there are necessary clothes in the box" the way a child would, by encountering different examples in contexts that slowly but surely clarify the difference between what a grammarian calls an attributive adjective and a predicative adjective.
But when you ask for help on Duolingo, you're asking other learners who have, to some extent, shortcutted the Duolingo system by using some formal grammar to figure certain things out by reference to things that they already understand, without doing thousands of exercises until things started to make sense. But the Duolingo itself teaches relatively little actual grammar, and most users never get to read those tips & notes anyway. So the answers to your questions are going to tend towards using grammar terms to try to answer your question, unless a simple example or two will suffice instead.
In this case, I'm pretty sure that you know enough grammar to be able to tell that there's a verb in Tá éadaí riachtanach, and there isn't a verb in "Necessary clothes", so Tá éadaí riachtanach obviously doesn't mean "Necessary clothes", because that .
The thing that you have to figure out is whether "necessary" comes before or after "clothes" in English. As Tá éadaí riachtanach is a complete sentence, and "there are necessary clothes" isn't, then "Clothes are necessary" makes more sense as a translation. You don't need any grammar to figure that out, you just need to be able to tell when a sentence is complete, or if it needs something else to complete it. You can use grammar terms to make that explanation more specific, but that's only a shortcut for people who aren't allergic to grammar. You'll have to do more practice without that shortcut, but that's true of anything - from learning to cook, to using a smartphone, to driving a car. You can either teach yourself from scratch or you can take a shortcut by having certain things pointed out to you, instead of figuring them out for yourself from scratch. You've probably never even paid attention to the fact that sometimes "necessary" comes after the thing that it describes ("clothes are necessary") and sometimes it comes before ("there are necessary clothes in the box"). You don't need to have a "vast knowledge of grammar" to see that there's something going on there, you just need enough examples to make sense of it for yourself. Once you've figured out that there is something going on, you might as well label it, so that you can refer to the same phenomenon when it crops up in other circumstances.
Bear in mind that you'll need to learn thousands of new words in a new language anyway, so learning a couple of dozen grammar terms to help you get a keep track of what's going on in the process of speaking another language is often a smart investment, but it certainly is possible to learn another language without doing that - people have been doing that for thousands of years.
I understand what you mean about Duolingo using spaced repetition to present different instances. The problem I'm having is that I'm not gaining insight from this method. I'm just unable to choose which instance is appropriate.
That's why the further down the tree I go the more time I've ended up spending reading comments trying to understand why a particular instance was used. I'm not able to decipher it without explanation.
It's not that I don't value grammar or that I'm averse to it. Quite the contrary, I perceive it to be the solution to the problem. But learning it would require a separate time commitment I can't make for practical reasons.
The only explanations I've really understood things from have been those made by contributors like yourself, Scilling, Galaxyrocker and anyone referencing grammar as the tool to use to actually understand what's going on in the sentence. I have to google the terminology to follow the explanation but eventually sort of get to some level of understanding.
It's just the time that takes me is too long. I could obviously learn grammar, there's bound to be the resources. But I know my kids had five years of it to become proficient, I don't have that sort of time available prior to learning Irish as well.
I wish this method of just being repeatedly presented with words and sentences would simply accrete somehow into meaning for me, but it hasn't since 2016 so I doubt it going to start now. I'm building a database of vocab words alright, but they're just character strings I've memorised.
What use is a database if you've no means to accurately query it and organise its output?
You're also right that people have been learning languages for thousands of years without needing grammar. People like me have also been failing to learn languages for the same amount of time. Most people I know gave up on Irish. Like a lot of my family (though some are native speakers) and friends I divided Irish in two, spoken and written. I gave up on spoken a long time ago tbh, I just can't hear enough words in the stream. I thought I could get to grips with the written version of the language though. I can see a maybe route with grammar as a sherpa, but it's up Mt Everest with no oxygen. No, I think I have to be realistic, this is beyond me.