I haven't eaten there, but I believe The Purple Onion in Tarmonbarry is very good. I've eaten in the pub next door (Keatings maybe? My memory fails me just now) and it was good pub grub. Its a fabulous little entrance into the town too -over the Shannon and on a good day, its stunning. I know whenver we've driven North-West we've passed through it, so I'm guessing its on the main route.
my boyfriend drove miles out of the way before so we could stop at the place phibbleberry mentions, Keenans in Tarmonbarry to get food there, have to say it was a gorgeous location and some of the nicest seafood chowder i tasted.