Obviously the Henry I had is not the sponsor of your ancestor. He would't have been around in 1855. I did find a Henry in 1901 Census, aged 71. Would have been in right age bracket. He had son Henry, born abt 1878, and a grandson, also Henry, born 1900. That could be "my" Henry. Do you have any other Christian names. I shall be in Carndonagh area in the next few weeks and will see what I can find out. I have relatives in Benduff. As for the Brochan House, I don't think you'll find anything online although there must have been some paperwork for the Commerative Plaque to be put up. Will try and find out and if anyone knows nickname for Doherty of Benduff.
I appreciate what it's like, trying to research Doherty in and around Carndonagh. My grandfather was one of them, and his parents were, as far as I can find out, both Doherty. It's like looking for a needle in a haystack. I wish I'd started when my mother was still alive, she knew who everyone was, and how they were related.
Will be in touch if I get any info
Ena