From Roscommon town to Dublin Airport is about 100 miles or 162 kilometres. Depending on where specifically in the county of Roscommon you are going to, the distance would differ, but not by a large amount.
Dublin Airport, Co Fingal Ireland to Ballinafad, County Sligo, Ireland: The distance between the start location and the destination is 114mi, (184km), and will take approximately 2 hours 18 minutes...