Song notes from the CyberHymnal.com. The words are attributed to Dallan Forgaill, 8th Century (Rob tu mo bhoile, a Comdi cride); translated from ancient Irish to English by Mary E. Byrne, in "Eriú,"Journal of the School of Irish Learning, 1905, and versed by Eleanor H. Hull, 1912, alt.
The tune is called"Slane" and is of Irish folk origin. Slane Hill is about ten miles from Tara in County Meath. It was on Slane Hill around 433 AD that St. Patrick defied a royal edict by lighting candles on Easter Eve. High King Logaire of Tara had decreed that no one could light a fire before Logaire began the pagan spring festival by lighting a fire on Tara Hill. Logaire was so impressed by Patrick's devotion that, despite his defiance (or perhaps because of it), he let him continue his missionary work.