Hello Gavin, I also have ancestors from Watergrasshill. We were in Ireland last fall and visited Watergrasshill. I went in the Church of the Immaculate Conception and saw windows that were dedicated to some O'Keefes from Ardnageehy. My gggrandfather was Philip Moore who was supposed to be from Watergrasshill. He married a Bridget O'Connell who was the daughter of Daniel O'Connell and Bridget O'Keefe. Have you run across any of these names in researching your O'Brien line? The area must have been really small around the end of the 1700's and the beginning of the 1800's. I am tempted to write to any people in the area with the surnames that are the same of my ancestors.
Stay safe and stay well from this virus.
Kathy in California