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I presume your ancestors were Roman Catholic. Unfortunatley, the RC parish records for Drumhome start in 1866. (Almost all RC parish records start around 1850 or later). I searched the subscription site Roots Ireland and there were no Kearns/Cairns baptismal records with father John in the Drumhome area.
I also did not find a John Carirns/Kearns in the Griffiths Valuation of 1857 for Drumhome parish Co. Donegal.
You may want to add Your great-great grandfather Patrick to our XO Chronicles site https://irelandxo.com/ireland-xo/history-and-genealogy/ancestor-database
This Ireland Reaching Out Insight may be of interest https://irelandxo.com/ireland-xo/news/irelandxo-insight-how-dna-can-help-you-connect-ireland