If you get on at Lucan and exit at Blanchardstown, and then go back the same way, you get hit 4 times for a short trip, and if you're unlucky enough to have to go through the M4 toll on the way up, add another €5.40 (return), but if you get on anywhere south of the bridge you can drive all the way to Wexford and back (...give or take!) on superb roads with no toll charge! Rant over!