"A boy needs water."
Translation:Teastaíonn uisce ó bhuachaill.
This word order is just what it is, even though it is not the standard VSO.
Teastaíonn + object + ó + subject
And then you have the exception with the verbal noun:
- Teastaíonn uaim snámh I want/need to swim
(The question "why?" is usually a hard one with languages. Life frequently is easier, when you are able to accept, that there are not necessarily any good answers. Learn like a three year old: Just accept it as it is, and don't mind the grammatics.)
...the 'subject' of this sentence is 'uisce', not 'bhuachaill'. 'Bhuachaill' is the object of the preposition 'ó'.
It may be clearer in the construction 'tá uisce ó bhuachaill': 'water is,' 'there is water'... I tend to visualize this like maths:
(there is water) <--> (boy)
The trick is understanding the <--> part. If the preposition is 'ag', the boy possesses the thing that is paired with 'tá' (the water). If the preposition is 'ó', the boy wants or needs the water (or whatever other thing is paired with 'tá').
I think the sentence literally means something like "a need/want of water is from a boy". So it is sort of an irregular sentence, with "boy" as the OBJECT instead of the SUBJECT. The format of a sentence is still the same, but the subject and the object are not what an English-speaker would expect them to be.
The beginnings of some words change depending on how they are used and the words preceding them in the sentence. There are two main types of changes:.lenition is the "softening" of the first letter and it is written by inserting the letter 'h' after the first letter, e.g. 'bean' becomes 'bhean'; eclipsis is inserting a new letter before the first letter, and only the new letter is pronounced, e.g. 'cóta' becomes 'gcóta'.
There are very good explanations of lenition and eclipsis in the 7th and 8th lessons in the Irish tree (these lessons are called Lenition and Eclipsis; they are just before the first checkpoint). Please read through the notes in these lessons, and perhaps take a look at some of the discussions about them.