I concur with Róisín. Brian O'Higgins composed the song in English. I don't know of any evidence that he based A Stór Mo Chroí on any older song. I am still seeking the relatively recent translation to Irish by Jackie Mac Donnchadh (recorded by Celia Ní Fháthartha accompanied by Catríona Ní Dhúgáin(clársach/harp) and Seosamh Ó Flatharta.