Hamill, McCue, Callan

My 3x great-grandparents, James Hamill and Alice McCue were both born in in Ireland in either 1817 or 1818. 

  A profile written in 1915 about their son, Patrick Hamill, states that he was born on August 15th, 1850, in County Monaghan.  My 2x grandmother, Mary Elizabeth Hamill, was born in either 1848 or 1849.                       The family immigrated to the U.S in 1851, living first in Morris County, New Jersey, then LeSueur County, Minnesota in 1859.  

Somewhere along the way their surname spelling became 'Hammel.'

I have found the Hamill surname throughout County Monaghan, but have no idea which line and which parish they come from.

Another clue from the profile of Patrick Hamill: an uncle from his maternal line, Thomas Callan, was a wealthy barrister in Ireland, and received an education in France that included military instruction.  Thomas came to America during the Revolutionary War, bringing his wealth and military training to serve (under an assumed name)  in the command of George Washington. Thomas remained in the U.S for 30 years before returning to Ireland. 

I would appreciate any help or suggestions.