Welcome back to ireland Reaching Out!
I assume your ancestors were RC. If they were not, please advise.
Finding RC church records in Co. Donegal before 1850 is very difficult and the records for Inver RC parish do not start until 1860/1861 so no marriage or baptismal records would exist for your ancestors.
I checked the Tithe listings for Inver parish but found no Kerrigan records.
The 1857 Griffiths Valuation head of household for Inver civil parish did list four Kerrigan records so possibly they are connections. http://www.failteromhat.com/griffiths/donegal/inver.htm