Welcome to the IrelandXO Roscommon Community!
As your questions are rather particular (and pre-19th century) the Roscommon Historical and Archaelogical Society would probably be your best bet for tracking down detailed information on this historical family.
The County Roscommon Heritage and Genealogy Centre in Strokestown would also be able to advise on recommended tour guides / car hire when planning your visit. (Public transport is limited and won't facilitate your needs I imagine).
Here you can also find a list of sites/buildings that belonged to the O'Connor Don as well as archival and printed sources which you may wish to peruse at the National Library of Ireland / National Archives should you be arriving via Dublin.
Certainly, any visit to County Roscommon should take in the Strokestown Famine Museum. You might also like to check out some other generally interesting Roscommon destinations here in our Local Guide.