it's from a poem by the blind poet Rafferty. There is a story by lady Gregory that she was visiting a hospital ward and overheard an argument between two older women about who was a better poet, Rafferty or another gentleman who's name escapes me. The argument was won when an even older women in the ward started singing this song by Rafferty, Bridin Vesey (as it is commonly known) and sang it all the way through many verses, proving that Rafferty's work was better known, regarded and loved by the people.