Phillipa, thanks for bringing more info to the surface on this subject. I combed through O'Hara's autobiography and found more on the song, particularly this:
"When I left school, Joan [Mary's sister] was established at the Abbey Theatre as a promising young actress.... While acting at the Abbey Theatre, she also did occasional radio work on commercial programmes (there was no television in Ireland until 1962). One day she mentioned to the radio producer that her youngest sister sang and played the harp. He expressed interest, and it was arranged that Joan should bring me along for an audition. I sang my entire repertoire: a handful of songs which included Joan's "The Quiet Land of Erin"...."
Apparently, her older sister Joan wrote the English version lyrics to Sean Mac Ambrois' song.