I encoutered the following online:

"Thugamar Féin An Samhradh Linn is a traditional Irish tune sung on May Day (Lá Bealtaine). Edward Bunting—a 19th century music collector—said this song “is probably extremely ancient” and was sung in the Dublin area around 1633. Even so, Pádraigín Ní Uallacháin says it dates back to 1745, yet Mary Devlin (author of The Lost Music of Ireland) claims it was 1726, so the origin is rather vague."

I distinctly recall reading that when King James Ii rode through Dublin on his way to the Boyne the crowd serenaded him with this song. Can anybody confirm?