I think there was a more practical reason, I think he was running out of Irish conscripts and needed more bodies to put between D.C. and the Army of NoVa.
Something I have trouble understanding is why so many Irish fought for the North. After escaping the tyranny of the English, you'd think they'd flock to the southern banner.
My best guess is this; the Irish faced massive discrimination and hatred from the native yankess, and were desperate to prove their worth and mettle to their adopted nation.
My own ancestor, Patrick Slain, was in the Union Cavalry. I just hope he never killed a southernor.