Although 'children' is usually the best translation, it has always been a singular noun. It is thought to come, from Old Irish from Old Welsh plant from Latin planta. Its original meaning, sometimes found even in Old Irish, was 'plant', hence 'offspring' hence 'children'.
It is quite normal in Welsh for nouns to acquire plural meanings, such as moch 'pigs' from the same root as Gaelic muc 'pig', as you will soon discover if you do the Welsh Duolingo course.
Changing p to c was normal in early borrowings into Old Irish as there was no p in the language - which is why there are still no words in Gaelic beginning with p apart from borrowings, a couple of words of unknown origin and the word piuthar which is really weird. Then nt changed to nn in Old Irish, so this is how plant became clann.
I should add that there is an important difference between Gaelic on the one hand, and Welsh and English on the other. While Gaelic clann, Welsh plant and English people all have singular origins and all look singular, the Welsh and English words are treated as plural
The people are (plural verb)
Y plant bach 'the small children' (no lenition that you would get with a feminine singular noun)
but the Gaelic word is treated as singular
A' chlann bheag 'the small children' (lenition on and after feminine singular noun)
Y blant fachwhat it would be if this feminine noun were treated as singular.
Edit: People have asked on several sentences what the corresponding singular word is. Normally, of course, plural nouns in Gaelic come from the singular, so you can easily get back to the singular. Not so in Welsh. Lots of words are plural to begin with and you have to add an ending to get the singulative (as they call it), for example
- plant 'children'
- plantyn 'child'
So the corresponding process in Gaelic is
- clann 'children'
- clannyn 'child'
Except, of course, that it isn't. We don't do that sort of thing, and we don't have a y. So we are stuck. There is no singulative.
Note on gender: all words discussed here, from planta to clann were feminine. Of course you can't tell in Welsh because it is plural. Plantyn is masculine as it has a masculine suffix.
I would use leanabh up to about 4, rather than just for 'baby' as they say on this course. Joanne says they do accept child for leanabh. A slightly older child is a pàiste. There is, however, no agreement about where the exact category boundaries are. Joanne says she only uses leanabh for baby and pàiste for a bit older than that. I'm not really sure there is a word that corresponds with child exactly, in its widest sense. There is a word duine-cloinne in the dictionary but I have never heard it and it can also mean man-child or human child (presumably as opposed to a fairy child).
I think if talking about a specific child you would always be able to find an age-specific term, or a gender specific one.
I'm not quite sure I follow you, but a' chlann means 'the children'. Clann on its own is just 'children'. 'Child' would be translated as pàiste or leanabh. While many Gaelic nouns form their plural with a na before them, this isn't the case for 'clann'. I hope this helps? :)
This is much more complicated than it might at first appear. First of all, what do you mean by 'sounds like an l '. There are perhaps eight ways to pronounce an l in Gaelic, including dialect variations, and just as many in English. I am saying this because I cannot hear anything that sounds like what I would call a 'normal l '. But I can hear something that sounds like what is often called a w. The thing is that l is often pronounced as a w, for example in talk which is usually pronounced tawk and Falkirk which is normally Fawkirk, except for people who do not know how to pronounce it. Similarly this often occurs in Canadian and some other dialects of Gaelic. Most people would say that clann (usually in Gaelic and in this recording) is pronounced to rhyme with English clown. However there is probably some nasalization on the vowel clõwn that you are hearing. That means that you are hearing two bits of n (the õ and the n) and the w in the middle that you are registering as an l.
But however complicated I make this, it does not matter how you interpret it. These words are pronounced in the standard way on this recording, and provided you pronounce them is a similar way you will be fine.
As for the why, which is the main question you asked, the origin of clann is well known. The second part of this post shows that it come from something like Welsh plant. We can assume this word existed in primitive Irish and it was quite normal for nt or nd to change to nn in Old Irish. This geminated n (as they call it) would have been a bit longer or even sounding double, and this could have led, ultimately to the pronunciation you are hearing.
The ann is more complicated, simply because no one knows where it comes from. The old word for 'in' is an (cf. Irish i(n), Welsh yn, French en, German in etc. etc.) The reason it changed to ann an is not obvious and is disputed, so we cannot say for definite where the nn comes from. What we can say is that wherever it comes from it is pronounced to rhyme with clann.
To summarise, the weird nn in clann is because it comes from something like Welsh plant, and the weird nn in ann is probably just a copy. D
Thankyou for a most interesting reply. You are quite right that in standard English, when you think about it, we often pronounce 'l' as 'w'. I find etymological explanations fascinating; they make you realise that words contain within themselves time capsules (or even wormholes into the past). My next question along these lines would be: why does a word that means 'small' (beag) come to mean 'big' in English!
Nobody claims that big comes from beag that I know of, and if they did I wouldn't believe them. But the reason they don't isn't the absurdity of the idea but the historical bias against Celtic languages. They will accept all sorts of nonsense if it does not involve Celtic, for example that wee comes from a word that is related to weigh and means 'weighty'. Using the sort of magic that you allude to, this then comes to mean 'not weighty' hence 'small'.
There are in fact two issues with big. One is that no one knows where it comes from. The other is that almost almost all words that mean 'big' in a range of languages have a broad vowel in the stressed syllable, presumable because you have to open your mouth wide - to make a 'big' mouth:
mòr, grand, large, enormous
Correspondingly you get slender vowels in words for 'small':
beag, petit, tiny
Against this, big and small (of dubious etymology, having apparently meant something else before) seem to be the wrong way round and are unique to English and Scots. D