Jim,
The source for all the church records is the Moville Presbyterian marriage register. Copy held in PRONI Belfast, ref: MIC1P/241.
19th June 1860 William Foster, full age, bachelor, carpenter, ur Moville s/o Francis Foster, farmer; Jane Baird, full age, spinster, no occ recorded, ur Moville, d/o Samuel Baird, baker. Witnesses David Fraser & Thomas McCandless (Thomas?s surname not fully legible, may not be correct).
19th April 1870 William Foster, full age, widower, carpenter, ur Moville, s/o Francis Foster, farmer; Anne Lynch, spinster, full, no occ recorded, ur Moville, d/o Wm Lynch tradesman. Witnesses David Foster & Fanny McAllister.
28th June 1861 David Foster, full age, bachelor, farmer, ur Ballysallan, s/o Francis Foster, farmer; Jane Montgomery, full age, spinster, no occ recorded, ur Ballynalley, d/o Joseph Montgomery, farmer. Witnesses George Hemphill & James Foster.
18th Mar 1862 John McIntyre, full age, bachelor, boot & shoe maker, ur Moville, s/o Daniel McIntyre, farmer; Polly Hemphill, full age spinster, no occ recorded ur Carrownaff & Moville d/o William Hemphill, farmer. Witnesses George Hemphill & Annie (surname illegible).
10th Sept 1889 Alexander McDowell, full age, bachelor, Occ RIC (Royal Irish Constabulary), residence Moville, s/o Hugh McDowell hotel keeper; Rebekah Jane Foster, full age, spinster, no occupation recorded, ur Moville, d/o James Foster coachbuilder. Witnesses John Kerr & Sarah Foster
4th March 1902 Robert Swann full age, bachelor, grocer, ur Moville s/o Wm Swann, farmer; Anne Foster, full age, spinster, no occ recorded, ur Moville d/o James Foster, coachbuilder. Witnesses Samuel Swann & Elizabeth Polly Foster.
4.4.1905 Archibald Beattie full age, bachelor, boat builder, ur Moville, s/o Thomas Beattie boat builder; Polly Hemphill, full age, spinster, no occ recorded, ur Carrownaff, d/o George Hemphill, farmer. Witnesses Wm Beattie & Elizabeth Polly Foster.
5th Feb 1907 James Foster, full age, bachelor, coach builder, ur Moville, s/o James Foster coachbuilder; Lizzie McNeary full age, spinster, no occ recorded, ur Carrownaff, d/o James McNeary, coachman. Witnesses Thomas Johnson & Mary McNeary.
25th Jan 1921 William Beattie full age, bachelor, boat builder, ur Moville, s/o Thomas Beattie boat builder; Elizabeth Polly Foster, full age, spinster, dress maker, ur Moville, d/o James Foster coachbuilder. Witnesses William John Beattie & Rebecca Foster McDowell.
Research in the early part of the 1800s is very difficult and it is normal to struggle at the point you have reached. My feeling is a that both families moved into the area in the earlier part of the 1800s (not necessarily from very far away) but that won?t make your task any easier.
I assume you know that because the families were Presbyterian, they most likely arrived in Ireland in the 1600s as a part of the Plantation. Curiously I did some research on Hemphill in Co Antrim for someone else recently and I noted that the name was quite rare in Scotland. Your Hemphill ancestors possibly came from somewhere like Ayrshire. (That wouldn?t be too surprising a conclusion as large numbers of people from Ayrshire settled in Co Donegal then). I had a quick look at the 1841 Scottish census. There were only 57 Hemphills listed. 14 were in Ayrshire and 16 in Glasgow & 12 in Edinburgh, but the Glasgow ones wouldn?t have been there in the 1600s. They would have moved in from some rural location as Glasgow developed as a city. Ayrshire stands out as a location where Hemphill was fairly prominent. That combined with the historical information about the origins of plantation families in Donegal makes me think that might be where they originated. But take that as speculation, not proof.
Elwyn