You don?t say what denomination your family were but I assume RC. Pettigoe is in the civil parish of Templecarn, but the RC parish of Carn. Unfortunately that parish has no baptism or marriage records earlier than 1851, so you won?t be able to trace any of the baptisms etc from that source.
Looking at Griffiths Valuation for the area, in 1857, there is only 1 Dowd farm listed. That was for Owen Dowd in Tawlaght, where he had plot 11 which was a 40 acre farm. That?s on the right hand side of the modern R233 which is the road from Pettigoe to Lough Derg, about half way along.
This is the only Dowd in that townland in 1901:
http://www.census.nationalarchives.ie/pages/1901/Donegal/Pettigoe/Tawlaght/1171185/
I had a look at the tithe applotment records which record those with land in the 1820s/1830s but there don?t appear to be any for the area around Pettigoe. (They haven?t all survived).
Probably your best bet will be to check the local graveyards and to enquire locally about any Dowd families.
Though Pettigoe itself is 95% in Co Donegal a small part is in Co Fermanagh and the Dowd family may have lived in Fermanagh, with Pettigoe just as their postal address, so you will need to bear that in mind when checking records. (I can see several Dowd families in the 1901 census in Backwood and Clonaweel, both of which are townlands just a short distance from Pettigoe, but in Fermanagh. You may need to check both counties records).
