1) Don't be too harsh on Meagher, the Irish Brigade was poorly used at Antietam and Fredricksburg, the latter in particular where they where asked to take on a dug in enemy with excellent fields of fire (see Longstreets comments).

2) There is an alternate story (which I am am trying to locate a refrence) that this song actually had origin as British propaganda, trying to discourage emmigration at this point as they feared now too many were leaving (someone has to work the Landlords ground and pay rents you know) and also concern that a lot of Irishmen were now getting a lot of Military experience and this would be a problem in the future.