"Siúil a Rúin" is a traditional Irish song, sung from the point of view of a woman lamenting a lover who has embarked on a military career.
The song has English language verses and an Irish language chorus, a style known as macaronic.

Live performance: https://www.youtube.com/watch?v=M9Jc2VHGQbY

Performed by Clannad
Album: Dúlamán (1978)