My great grandmother's name was Katherine Murtha, she was married to Emil Chagnon, their daughter Katherine Louise Chagnon married Walter Joseph Kellaher, their daughter Joyce Marie Kellaher (my mother) married Wilfred John LaFrance. I know that Grandma K and PopPop were "always" in West Haven Connecticut. My mother was born in West Haven, CT. Our family migrated to the west coast and, although I was born in Connecticut I grew up in Washington State.
Is this the same Katherine Murtha as the one you mentioned from Westmeath Co. Ireland? Family history has her coming from Ireland on a potato boat sometime around the turn of the century.