In his book Love Songs of the Irish, Mercier Press 1977, James Healy gives this overview...

The original song has been attributed to a priest, Oliver O'Hanley (c1700-1750), written in praise of a County Limerick beauty named Nelly O'Grady, but the tune may be much older than his song. Some attribute it to a seventeenth century bard from Benburb named Muiris Ua Duagain, but in the absence of documentary evidence anything we know about this beautiful song is speculation.