"We are only boys."
Translation:Níl ionainn ach buachaillí.
It is word soup as far as Irish is concerned, a random collection of words that don't mean anything in particular when used together.
Ach generally means "but". Ach is used with níl when you want to say "only", and it is used with amháin when you want to say "except" (but sometimes "only" and "except are interchangeable in English).
only is a negative adverb, and you use nil, not tá to express that in Irish.