I just learned this slow air from the playing of Matt Molloy. It's another great track from Heathery Breeze. It's a short, simple tune, and Molloy plays it in D, so it's easy to pick up. It's beautiful on both the flute and the whistle. I've seen it listed as a waltz, but I think it would be a mistake to play it that way, in spite of the time signature. Molloy's version really makes the most of the freedom which comes from playing it as an air. The tune is in A mixolydian.
Like a lot of slow airs, this one comes from a song. You can hear the song, or a version of it called "Idir Áird Mhór Is Eochaill," anyway, on Danú's All Things Considered. I also found a transcription and translation of the words that Ciarán Ó Gealbháin sings. This review says that he got it from the singing of Éibhlis Bean de Paor from An Sean Phobal. The melody of Molloy's track is pretty much the same, but the places in the title are different.
What does "Idir Deighric 'Gus Breo'" mean? Well, "Idir" is "Between." "Gus" appears to be a contraction of the Irish "agus" (and), and Deighric and Breo are mountains in Co. Waterford. The Waterford County Library says that the names "defy analysis, and are evidently, like many mountain and most river names, of great antiquity." So my rough translation of the title is "'Tween Deighric 'n' Breo."