how far is the nearest airport to Roscommon,
It depends where in Roscommon you are.
Knock is the closest airport.
It is about 20km from Ballaghaderreen, but 100km from Athlone.
From Roscommon town, it's about 50km.
From Athlone, you may find Dublin Airport as handy as it is only 130km and has direct bus services.