Anybody want to have a go at the above? Or should I just resign myself to the fact that it is untranslatable?

A literal translation won't have much meaning in Irish, but here's my attempt at it. The tricky part is translating the second half f it, as clearly "your" is implied, but not stated, presumably because it's an established phrase. I don't see any other way other than to be very literal to translate this, but of course it comes out pretty silly then.

A bhreagadóir, a bhréagadóir tá to bhristí trí thine (Oh) liar, (oh) liar, your pants are on fire

Translating such established phrases literally rarely works unfortunately.

Seems to me the equivalent wouldn't be a translation so much as whatever rhymes Irish schoolchildren use to mock liars. I haven't a clue what these might be, but I did find this saying from Tír Chonaill:

An té atá bréagach, tá sé bradach. "A liar is a scoundrel."

Always better to use an established phrase rather than literally translating one from language to the other

Exactly something rhyming would be good, because it's being used as a taunt (should have specified that, mo leithscéal).

I could throw in your phrase a Dhomhnaillín Breac, or try and form a rhyme out of it!

Bréagach, bréagach, is tú an bradach ??

er…The saying from Domhnaillín Breac already rhymes.

An té atá bréagach, tá sé bradach.

I know, but it's more of a seanfhocal than a taunt that a child would say to another child, which is what I'm trying to come up with

Not in Munster, is muar an trua.

In a Tintin comic there was an exchange something like this:

-"Béarfaidh mise greim ort!"
-"Beir greim ar mo bhroim!"