According to the Monaghan county historian and author of "The Monaghan Story", Peadar Livingstone, The surname MONAGHAN is probably a local family name.
The Hearth Money Rolls, maybe in the 1600s, show 9 Monaghans in Monaghan town.
The root of the name seems to be "manach", a monk.
In 1976 voter rolls there were 29 Monaghans in the central part of the county, 37 in the west, 24 in the central, and 16 in the south.
Pat T.
http://www.angelfire.com/my/tray