Thank you Rodney! Yes, I actually asked this question because I came across "is" used instead of "agus" in natural speech – no poetry involved – but using the short form in order to keep the rhythmic structure of a verse makes total sense too. So yes, thanks for adding this!

You will often see 's in poetry or songs to get the desired meter or beats, although that probably isn't what you were asking about.

Funny, I didn't know questions can "expire" here, whatever that means... Anyway, I'm in no rush so if the absence of the "Best Answer" sign doesn't make you too sad (alas, it seems like it doesn't work with these "expired questions" — and, just for the record, I wish I could use it!) then it doesn't matter. Seriously though, thanks a lot for your answer! Very glad to know that "agus" is never wrong, that's a relief. Well, I think then it makes sense just to pay more attention to the phrases where "agus" ≠ "and"... Thank you, knowing there's probably no clear rule, that's very helpful.

I didn't manage to answer your question before it expired, but anyway... I don't know of the existence of any particular rule concerning "agus" and "is", I would say that they're pretty much interchangeable ("is" is just an abbreviated version of "agus"), although there are some cases where you'd be less likely to hear "is", such as when "agus" introduces a clause (e.g. "Tháinig sé isteach agus é fliuch báite"), and there are some turns of phrase (often in which "agus" doesn't translate as "and") where you would rarely hear a full "agus" (e.g. "tuairim is troigh ar airde"), and the same would go for things which generally go together (e.g. "cith is dealán"), though "agus" would never be wrong.