It is a nice song indeed (sometimes I hum it for my niece, I don't want to scare her by actually singing it).
The fact that the words are English is not surprising, several "traditional Irish" songs are translated in English. In act several songs in the top of the "Irish Traditional Song List" (such as "Danny Boy", "It's a Long Way to Tipperary" and "Molly Malone") aren't even written by Irish authors.