Hello
Re: the line
For Sarsfield is the word tonight and Sarsfield is the man."

Any reference to Patrick Sarsfield whether poem, song or whatever
would most likely contain the line

The British garrison in Limerick had a password to gain entry to the
town. The password was changed frequently. The British believe it or
not, choose "Sarsfield" as the password.
When he arrived the soldier challenged him with "What is the word"
And the answer he got was.......

Regards

Frank
(I'm afraid the detail is sketchy, but you should be able to find the
full account somewher on the net.) Sorry cannot help with the song