I wonder if it's related to the film of the same title, Young Cassidy. TCM.com says this:

Rod Taylor stars in the true story of Irish playwright Sean O'Casey, Young Cassidy (1965), directed by John Ford and Jack Cardiff, also starring Julie Christie, and Maggie Smith.

The movie trailer doesn't have the song. Anybody want to transcribe the Clancy recording from the YouTube video?

-Joe-