Traditional folk tune, had a horrible discovery just now. If you google 'Slane' on it's own, you just get the Irish plot of land, not the music.
I first discovered at church, through a song called "Lord of Creation", that the tune to "Be Thou My Vision" was, in fact, a traditional Irish tune to which the words of Be Thou My Vision had been placed. (An overly enthusiastic "modernity" type had put up, at one place I went, "Be You My Vision", but that's another story.)
http://www.digitalsongsandhymns.com/songs/9021 (http://www.digitalsongsandhymns.com/songs/9021)
Not sure I'll ever forget 'Lord of Creation' either, as an elderly lady at church introduced Dad and I to it. Sounds silly on the internet, but some things you never forget, like that.
I typed out a first reply to this, but it devolved into a long and overly detailed recounting of my Catholic upbringing. Too much information. Suffice it to say, I was raised Catholic, and Catholics aren't allowed to have joyous music. Though it warms my heart that others have had better experiences.