Welcome to Ireland Reaching Out!
You did not mention the religious denomination but I will presume Church of Ireland.The subscription site Roots Ireland does have an 1827 baptismal record for an Isaac Barr July 1 1827 (birth June 24 1827). Father was Isaac and mother Nancy. The record was in the Moville Upper Church of Ireland parish. The baptismal records for this parish go back to 1804 but Roots does not have marriage records for the parish.
The Griffiths Valuation for Co. Donegal was published in 1857 and likely accomplished a few year before 1857. If your William emigrated before 1851, I'm not sure the Griffiths record is for him.
I searched the Tithe listings on the National Archives site and there were no Barr records in Moville Upper.
Let me know what questions you have.