I would go with the definition: an Irish worker. I'd guess it was a slang term to refer to the Irish laborers blasting out tunnels in rock for the railroads. It's Just like John Henry who did the same thing for the Big Bend Tunnel of the Chesapeake and Ohio Railroad in West Virginia. A lot of this work was done by Asian so-called "coolee" laborers as well. There were many premature blasts that went off.