The Great Western Greenway is a the longest off -road cycle and walking route (77km) in the country. It goes from Westport to Achill. Built mostly on top of a old railway line which was closed in 1937. I cycled part of it the other day from Newport to Mulranny. We rented bikes from Dave at Clew Bay Bike Hire and not only did I get a bike that almost free wheeled all the way but they also come and collect you at the other end so you don't have to cycle back. In fact nothing was too difficult for them, (booking is advised).
What I enjoyed most about it was the fact that we were up close to the landscape, car free and safe, It's an easy cycle even for the unpracticed and the views are stunning especially as you approach Mulranny and the Atlantic.
We had a lovely lunch in The Hotel Newport before departure and I believe Nevins is good too.