I am researching my husband's McDonald ancestors from County Monaghan also.Your mention of Hempfield, PA caught my eye as his McDonalds lived in Greensburg which is in Hempfield Township in Westmoreland County PA.
As far as I know though his first McDonald ancestor to come to the U.S. came about 1890.That was John J. McDonald born in 1867 in Shotts Iron Works, Lanarkshire, Scotland.His mother Bridget McCann, father Patrick McDonald, older sister Mary, brothers Patrick and Thomas were all born in Ireland.
Bridget and Patrick McDonald were married in Monaghan in 1854 according to the birth record of their son John J. McDonald.
I've found that Patrick's parents' names were Thomas McDonald and Jane Connolly.Bridget's parents were John McCann and Catherine Boylan.
That's all I have been able to find out so far, so it might be difficult to find out if they were related to your McDonalds, but I thought I would share this information with you anyway.
Also, I have had my son's Y-DNA done and am waiting on the results for last part of the 67 marker test.If you have had any McDonald Y-DNA results for your family, it would be very interesting to compare the results.