BPM, I am not aware of any gym in Castlerea. There is one nightclub and plenty of pubs, but I suspect people travel to find their entertainment. I wouldn't know about the rental market but I had a look in the local paper, Roscommon Herald, and they are advertising rooms to let but no price given. There is an outdoor swimming pool but most people go to Roscommon Town where there is a new indoor pool.
I have just asked my daughter who says not to expect city facilities as Castlerea is a small town. In her opinion the nearest decent gym would be in Carrick-on-Shannon but if your point of reference is Dublin then that would still be classed as a small gym.
Yes, Knockroe would be convenient for getting to Roscommon town and would take about twenty minutes. It is an up and coming town but as yet has not got enough facilities, but if someone works in Roscommon town then they can shop there etc.