There is no puiblic transport to Tallaght from Monasterevin. As for the commute, the best way to Tallaght is via Citywest, however, it can be badly congested from about 8am onwards. An alternative route is up through Saggart. The new 3 lane road from Naas has made things much better on the approach to the city and Tallaght is a good place to be commuting to from the N7 end of things.
Best thing to do is try the 'course' a couple of times to see how long it takes you at the various times of the morning.