I had always assumed this to be quite old but it appears it was written by Ethna Carberry (AD 1866 - 1902). The tune is older.

Here's the entry from the Traditional Ballad Index.

Roddy McCorley
DESCRIPTION: "Oh see the fleet-foot host of men..." who are hurrying to stage a rescue. "For young Roddy McCorley goes to die on the bridge of Toome today." They are too late. The song recalls McCorley's actions; he would not turn traitor even to save his life
AUTHOR: Words: Ethna Carberry
EARLIEST DATE: 1961
KEYWORDS: Ireland rebellion death execution
FOUND IN: Ireland
REFERENCES (2 citations):
Silber-FSWB, p. 324, "Roddy McCorley" (1 text)
DT, RMCORLEY*
File: FSWB324

Cheers!
Ian