I noted the following probate abstracts for Pearsons near Letterkenny:
The Will of William Pearson late of Letterkenny in the County of Donegal Yeoman deceased who died 8 June 1864 at same place was proved at Londonderry by the oaths of John M'Auley of Letterkenny Auctioneer and John M'Auley of Ballconnelly Farmer both in said County the Executors.
Pearson Anne of Letterkenny county Donegal widow died 5 December 1926 Probate Belfast 7 August to Anne Jane Pearson spinster. Effects ?28 19s. 1d.
Pearson Annie Jane of Letterkenny county Donegal Eire spinster died 2 February 1946 Probate Belfast 27 June to Arthur Alexander Andrews bank official James Alexander Moore merchant and James William Thompson bank official. Effects ?11819 9s. 6d.
The current Irish phone book lists 3 Perasons in the area including a Charles & Sylvia Pearson in Lismongahn, one of the townlands you listed. So it would seem worth contacting them.
I know of 4 Presbyterian churches in the Letterkenny area. Letterkenny 1st has baptisms from 1821(with gaps) and marriages from 1845; Letterkenny 2nd has baptisms and marriages from 1821; Letterkenny 3rd baptisms from 1836 and marriages from 1838. Trenta has baptisms from 1836 and marriages from 1838.
Some of those records are on the rootsireland site (subscription) though I am not sure all of them are listed.
There are however copies on microfilm in PRONI (the public record office) in Belfast.