Roscommon Mill =where was it?.

Is there a context for "Roscommon Mill"?  There were different types of mills: water, grain -grinding, cotton and timber. Any of these in Co. Roscommon could be referred to a Roscommon Mill.  There was a Mills family in Co. Roscommon.

I've not heard of anything 'Roscommon' in Co. Cork.  A check for 'Roscommon' in Irish places names doesn't give anything in Co. Cork.

Another place to check for heritage buildings, landmarks and item is National Inventory of Architectural Heritage (NIAH).  

I would second guess that Roscommon Mill is a mill in Co. Roscommon.  The universities have specialisations in various items: social history, buildings, language etc.  Their sites may be worth exploring.

Sorry, I can't help further.

Good luck