The route of the Grand Canal in Dublin has some great cycle routes. Between Portobello and the Grand Canal Dock, there’s a new segregated cycle route that is pretty good, apart from the needlessly-confusing traffic signals at every junction.
And a bit further west, between the 3rd and 12th locks (Blackhorse LUAS stop to Lock Road in Grange Castle), is an 8.5km stretch of the Grand Canal towpath that was refurbished a couple of years ago into a really nice shared pedestrian and cycle route. There’s lots of dog walkers, runners and cyclists using this route. However, in order to stop cars and motorbikes tying to drive along it, there are quite a lot of gates to pass through, which can interrupt the ride.