The tune to "Lord...", is called "Slane." A midi by Barry Taylor is here: Slane

The melody in Peacock to "Brown Girl" is similar in part only.

Americans are more likely to know the hymn as "Be Thou My Vision," sung to this tune and believed to be 8th century. Be Thou My Vision
"Be Thou My Vision" is attributed to Dallan Forgaill, 8th Century, "Rob tu mo bhoile, a Comde cride," translated to English by Mary E. Byrne, 1905, versed by Eleanor H. Hull, 1912. Data from Cyberhymnal.