I just read your thread. I'm sorry that it took me nearly four years to see it!
As this thread is about the Hamill's I am not sure if any of us can be of much assistance to you, but I will tell you what I know.
The Owen McMahon of Castleblaney that we referred to in this thread was my great-great-great grandfather. I do not know his date of birth , but his daughter, my great-great grandmother Mary Catherine was born in 1876.
You estimate that Ross McMahon was born in 1833. So unless my great-great-great grandfather fathered two children 43 years apart, it is doubtful that the Owen McMahon you speak of is the same man that was mentioned in this thread.
If I were a guessing woman, I would say that "your" Owen McMahon was either an uncle, or perhaps even the father of "our" Owen McMahon... and that Ross was perhaps a brother or cousin of my great-great-great grandfather.
Let me know what you find!
Bridget Hamill Carbonara