The calculated distance (air line) between An Muileann gCearr and Roscommon is approximately 35 miles respectively 57 kilometers.
By car or train, the actual journey to Roscommon is certainly longer, as only the direct route (as the crow flies) between An Muileann gCearr and Roscommon has been calculated here.
Air line (approximately)
57 kilometers
31 nautical miles