Royal Canal Walk, Dublin
If you happen to be in the Dublin 15 area (Castleknock/Blanchardstown/Clonsilla), you might want to talk a walk along the Royal Canal. It is not particularly strenous, though it is narrow and muddy. It is so quiet, you would not think you were in Dublin at all. The Dublin-Longford rail line goes parallel to the Canal.
I suggest you start off in Leixlip, which is on the River Liffey, on the Dublin-Kildare county boundary and keep on walking until Castleknock Marina, which is the 12th Lock on the canal. Co-incidentally, there is a fantastic bar at this spot, called (wait for it!) The Twelfth Lock! It is a lovely bar and let's face it, you will be parched for a drink and a loo break by then. There is a regular bus near here to take you back into Dublin City Centre. Or you can go back under the bridge to Castleknock train station to get you to Dublin