This Jacobite song, entitled "Seárlas Óg", was collected in Co Tyrone from the brother and sister Ned and Nancy Tracey by several collectors around 1910. A cylinder recording was made in Tyrone by Roger Casement's associate Michael Deegan but seems to have been lost. The tune was noted from the Traceys and arranged by Carl Hardebeck. Several contemporary publications of the words can be seen at https://www3.smo.uhi.ac.uk/oduibhin/bgilmore/furthernotes.htm as can Hardebeck's musical arrangement, and a modern authentic performance by Peadar Mac Gabhann. The song was heard in Tyrone by Roger Casement who drew Pearse's attention to it, and he rewrote the words removing any suggestion of a Scottish or Ulster Gaelic connection. I too wonder whether some version of it was known in Scottish Gaelic, but I have never heard one.