No, Peter - that's an old one! Apart from anything else, "buachaill maith" is "good boy" - so why would it become female only?

The main slang dictionaries (Grose, Partridge etc.) trace it to "mort" as from an Old Dutch word meaning "woman" and, eventually, "harlot". Earliest printed references late 17th C. Once common, then isolated in Ireland - particularly Dublin, of course. None of the Irish experts (Share, Dolan, Ó Muirithe) go along with a Gaeilge origin.

Regards

p.s. There is one suggestion of a possible Romany-Shelta root - but I know nothing of it.