We will be arriving at approximately 6 am on a Friday morning in May and plan to spend our first day and night in Dublin. Any suggestions on whether to pick up our rental car on arrival (our hotel has parking) or wait until the next day when we plan to head south to Powerscourt? Also, if the latter is best, where should we arrange to pick up the car (probably Hertz) in order to have the easiest time of it? Thanks for any suggestions you may offer.

Are you coming from the US? If so, you'll be tired and not used to driving on the other side of the street. Are you experienced at city driving? If not AND if your hotel is right in Dublin, you'll be surprised, particularly because it sounds like you might hit the city along with people driving to work. I would wait until the next day. Sorry I can't help with a pick-up location.

The Hertz in Dublin is on the South Circular Rd. Dan Dooley (my choice) and Europcar are in Ballsbridge, closer in, and within walking distance of, say, St. Stephen's Green.

Thank you both for your input. I think I'll wait until the following day to pick up the rental car.

I would definitely take the city bus (2.20 one way) or taxi if you have the big bucks back to the airport to pick up your car the next day. You'll avoid the city traffic (you'll thank me later!) which can be a nightmare. From the airport..just hop on the M50 and loop around and out of the city.
MUCH easier!

I second memichele's advice. Having seen the Dublin traffic when we visited a few weeks ago, I was glad we opted to stay outside of town and take public transportation into town for the one day we visited. We were staying near the airport and we found it so easy to drive to the M50 ring road and get wherever we needed to go.

Since you are going to Trim, do the rental car from the airport when you leave Dublin. It is an easy drive, just don't do it near rush hour.