Hi Plays,
Well you certainly seem to have done quite a bit of research! Isn't it frustrating not being able to confirm a link? I have found a man in Ayr, who I am sure is related to my family there, but I can't prove it! UGH!
However in your case I think that two O'Byrne families living right next door to each other in Partick is very telling, as I have found my Irish ancestors living very close to their other family members a number of times.
Have you tried searching for your family at
http://www.emeraldancestors.com/ . This is a subscription site and I have heard it is very good, but have not yet used it myself as I would like to get full value of a month's subscription, and have not yet found the time.
Griffith's Evaluation
http://www.failteromhat.com/griffiths.php has not one O'Byrne name under Leitrim, but there are three Partick O'Briens. Similarly there are no McMurrays, but plenty of Murrays. I wonder if it would be worth trying to cross reference the town names from the Patrick O'Briens with the Murrays and see if it can give you a township (in which both names are present) to look for? Just a thought, but I would bet you have already tried looking there.
As for the 'translation thread' I agree with everything the poster JAP has said, and I particularly agree that Barclay is probably a variation of Bartholemew. I have some Irish Bartholemews known as Bartley...not too far away from Barclay if you say it a few times with an Irish accent!
Let me know how you get on!
Nina