This is a traditional Irish song arranged by The Gloaming. It appears on their debut album with the same name.

From gaolnaofa.org

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.

Note: “féin” can be pronounced “hayn” or “fayn”, and “thugamar” can be “hugamar” or “hoogamar” depending on the speaker. Same with “Samhradh”, which can be “Sau-roo” or “Sau-rah”. For example, here’s a rendition by The Wolfe Tones (notice their alternate pronunciations compared to Mary’s below).

Others have recorded this song as well, though under different titles (and with different arrangement or additions of the lyrics——as it goes with trad songs): Pádraigín Ní Uallacháin recorded it as Amhrán na Bealtaine on her album A Stór ‘S A Stóirín, and Eithne Ní Uallacháin & Gerry O’Connor (a.k.a. La Lugh) recorded it as Beltane Song on their album, Brighid’s Kiss.

From Eithne Ní Uallacháin

A Gaelic Summer song, celebrating the joys of nature and youth. It was collected by Enrí Ó Muireasa from Owen Byrne Farney, Co. Monaghan and published in Céad de Cheoltaibh Ulaidh. The song was sung by young men and women on May Eve, while they carried around the “May Baby” a large doll dressed in ribbons. Bealtaine was the Celtic festival of Summer.