Harlequin,
If leaving from Dwyer street, I'd definitely suggest heading out the Slane road. I've been through the village before 8 in the morning and it never seems too badly congested - not sure about the evening.
I assume Railway street is out the Trim road (the tax office) so you could head down by the river and turn right at the lights (bypassing the town centre). Again, I don't know how congested it gets there in the morning..
If you are offered the job, bribe a mate into taking you on the drive one morning to see how long it takes.
Good thing about having the car is the flexibility it gives you should you need to work a bit late...