Hi Brie
What a fascinating ancestry you will have!
Now I think it is a case of joining the two up.
To revert back to your original query, it does sound as though the majority of your ancestors came from the more 'northerly ' Camlin. Perhaps they moved south and started naming their houses Camlin and even areas/roads/estates as Camlin (depending on their influence within the community!) I am of course guessing at this

- the documentation you eventually find will tell you the facts.
Personally I have always found it easier to work from present day back - I do not know if they had birth certificates in 1821 (any experts please help here!

) I think you may be back into the parish registers.
It may be worth liaising with aghadowey to see if you have any 'phibbs' in common.
Also on the Irish General page there are a couple of articles from Ticker who is the moderator here and they will give you some great sites to search on.
I couldn't see any other Camlins, but calling all Roscommon experts out there - please let Brie know if any different.

Sorry to generalise on lots of points, I wish I was a little more knowledable on Irish geography but not as yet!
Good Luck Brie and stick at it.

Su