FWIW there was a term "buckle" which in certain times - probably medieval - referred to a lady's pudenda. Now a playwright - particularly an Irish one - would most likely have knowledge of ancient terms, and who knows if it survived longer in Ireland.
Certainly when I was busy writing poetry and lyrics I had my head buried in all manner of word books like Brewers Dictionary of Phrase & Fable. Dominic Behan would have done similar IMHO.
And - sure - wouldn't he not be averse to a little joke at the expense of the naive? Given the reading of the lyric without dirty glasses it is a lovely gentle song. But that is the way subtle jokes work.

I used to sing the "Unquiet Grave" with a sound operated dancing flower next to me. It always got a huge laugh. It is the juxtaposition of the serious and the absurd, the surprise, the incongruous, that makes the humour.