I searched the subscription site Roots Ireland for all of Ireland and did not find any Donohoe/Donohoo baptismal records with father Patrick and mother with surname of Faherty.
Unfortunately it is very difficult to find any Co. Galway RC parishes with records back to the late 1700s. The only parish which has records is Tuam. Likely the children of Patrick and Sarah were baptized in a parish which no longer has records back to the late 1700s.
Have you considered DNA testing?