I've been asked by someone on a Facebook page if there are any guidelines as to when to use "agus" and when to use "is"? What I told her is, as far as I know, it's a matter of which flows best with the sentence, and it's just something you start getting a handle on as you do more speaking, but I also told her I'd double check and make sure there isn't some kind of rule (because, despite what my husband says, I'm not ALWAYS right!)

Redwolf

From what I've been taught, I'd say you're right, but it would be interesting to hear what a native speaker like Bríd has to say, since she probably makes the choice automatically.

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

In writing, I learnt that you use "is" in set phrases (set "couples of people or things"), and "agus" otherwise:

Romeo is Juliet, Diarmaid is Gráinne, d'oíche is de ló...

In speech, "agus" is oftened shortened to "s" or to "us", which makes it difficult to differentiate from "is"

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

I remember asking this question before. I read somewhere in one of the grammar books or in Teach Yourself Irish (Myles and Ó Cróinín) that 'is' comes between two adjectives and of course in set phrases. Though when I said that here, the reply that I got was that I was stating the obvious. Very often it can also come at the beginning of a line in a song in the guise of - 'S. Something I find a bit surprising in a way is that in storytelling they always seem to start a new sentence with agus. The number of times Séamus Ennis does this is overwhelming. And then there's how it's used grammatically in Irish in ways it wouldn't be in English, except of course in Irish English which, perfectly logically, is Irish transposed into English.

chan i gcónaí... dá mbeadh sé fíor, cha ndéanfadh na hÉireannaigh (nach cainteoirí dúchais iad) an méid sin Béarlachas agus iad ag labhairt Gaeilg
Is iad na seanchainteoirí Gaeltachta a aistreas a gcuid abairtí Gaeilg i mBéarla focal ar fhocal

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

That's why I can't give a grammatical explanation. I don't know which is which because when I say "agus" it is almost always abbreviated to either "is" or "gus" unless I want to say it for emphasis then I would say the full word.