yes, that's Fáinne Gael an Lae. I suppose that's yet another Irish-language lyric for me to type and post! And in Kathleen Hoagland "1000 Years of Irish Poetry", there is a translation by Edmunc Walsh, "At early dawn I once had been/Wher Lene's blue water's flow." Hoagland says the original dates from the 18th century.