I’m not sure why this tune isn’t listed here, it’s a gorgeous air written by Sean O’Riada. Derek Bell plays a lovely version on Musical Ireland CD.
Ag Criost An Siol
Translated as "to Christ the Seed", it is the title of a prayer, which Ó Riada set to music.
Some might tell you that it was to commemorate a papal visit in 1979, but Ó Riada died in 1971, so the claim is refuted.