Slane is an old Irish folk tune that has been set to several hymns. Notably "Be Thou my Vision" and "Lord of All Hopefulness." It is also very similar to a northern Irish folk tune "The Banks of the Bann."
The tune is named after Slane Hill,County Meath, where St. Patrick is said to have lit a candle (or bonfire depending on the source) in defiance of an edict by the local king that no fires should be lit before or while the festival bonfire was burning during the pagan festival of Bealtine. Bealtine corresponded with Easter in the calendar. St Patrick lit his fire on the eve of the Easter feast day in 433 A.D.