Peter, Paul and Mary didn't mangle the song; it had already been mangled in folk transmission in North America. The song was widely spread in the U.S.; I suspect it was brought in by Irish emigrants well before the Irish famine (hence the song Buttermilk Hill which is said to have been popular around the time of the American war of independence), and brought in again by successive waves of immigration - so there would have been lots of chances for the song to get around the N. American continent.

The Traditional Ballad Index https://fresnostate.edu/folklore/ballads/R107.html doesn't give any 18th century sources but cites:"EARLIEST DATE: 1876 (Waite) (but five broadside prints are listed in Edwin Wolf 2nd, _American Song Sheets, Slip Ballads, and Political Broadsides 1850-1870_, Library Company of Philadelphia, 1963, p. 78); a partial text is reported from 1823"

Among the citations is a 1927 recording by Chubby Parker, "Bib-A-Lollie-Boo" and a version collected by Alan Lomax with the words ""Shoo, Shoo, Shoo-lye". It would seem more likely to me that Peter, Paul and Mary got their nonsense version of the chorus from a pre-existing collection rather than hearing an Irish language chorus and making something up themselves.

There is more than one version of the Irish language chorus as well, so you will find both "Siúl a Rúin" and "Súil a Ghrá", using different terms of endearment.