Griffiths Valuation for 1858 lists William Armstrong as farming in Warrentown. He had a house, outbuildings and 5 acres on plot 6, as well as a half share of a house, outbuildings and just under 2 roods of land on plot 5. He was sharing there with Alexander Lawn. William also had a two sixths share in some nearby mountain grazing (plot 1) with some other local farmers. That farm today is beside Garran Lough, a mile or two west of Kilmacrennan.
You say the family were evicted around that time. The revaluation records would tell you when. The records for Donegal are not on-line but are kept in the Valuation Office in Dublin. If you e-mail them they may look the records up for you.
I tried to search to see who was living there in 1901 & 1911 but the census records for that and several nearby townlands appear to be missing.
I searched for a Bridget Armstrong death 1864 (when statutory death records begin) up to 1901, in the Letterkenny registration area. There were none. So either she died before 1864 or moved away from the area. The only Bridget Armstrong death anywhere in Co Donegal in that period was in 1892 in Ballyshannon, for a woman estimated to have been born in 1822. Ballyshannon is about 50 miles south of Warrentown.
I searched for a death of a William Armstrong, in the Letterkenny registration area 1864 ? 1901. There was just one. He died in 1864, est year of birth 1799. The name William Armstrong is fairly common in Ireland so it?s not necessarily your family. But you might want to order the certificate to find out. You can get a photocopy from GRO Roscommon for ?4.
You don?t say what denomination the couple were. Armstrong is almost exclusively Church of Ireland (in Donegal anyway) with the odd Presbyterian. Lawn is largely, RC, though there are a few Church of Ireland. So statistically William and Bridget?s may have been a mixed marriage.
If they, or their children were Church of Ireland then the bad news is that the parish has no records prior to 1881. (The earlier records were destroyed in the 1922 fire in Dublin). If they were RC, then the parish is called Termon & Gartan. They have no records before 1877. So either way it doesn?t look as though you will be able to trace the baptisms or parents marriage through church records.
There?s about 5 William Armstrongs in Co Donegal in the 1901 census but there?s no obvious way of knowing whether any of them may be Wm junior. There?s a fair chance that after his parents died and his siblings had left, he would have left the area. If he didn?t go to the USA or Australia, the next likely destination for folk from Donegal was Scotland.