Help me plan a realistic first day in ireland

Store your luggage
http://www.touristofficedublin.com/luggagestorage.php

Then go with what you already had planned.

Thank you! I believe I was updating my post as you replied I had not thought about storing the luggage and then backtracking to the airport, but all things considered that might be the best option for us.

As an alternative to backtracking, there is a baggage storage place right next to Heuston Station in Dublin, and car rental locations at Heuston as well. Here is information I got from my AirBnB host: "The bag storage I usually recommend is Tipperary House. It is adjacent to where the "Airlink" airport bus drops off (Heuston Station stop). You can search online for "Tipperary House Parkgate Street"."

With this option, you hop on the Airlink bus to Heuston Station. Check your bags at Tipperary House next door. Take a walking tour so you can just follow someone who knows their way around without worrying about the state of your map-reading and decision-making skills after the flight. Pick up your bags and a car at Heuston Station and drive (carefully) to Drogheda, which isn't too far.

I am an Ireland novice, first trip is this summer, but in the abstract it looks like this would be the easiest. Maybe someone who knows first-hand about getting out of Dublin from Heuston Station vs. leaving from the airport will chime in.

the baggage storage I was referring to in the link above is in the city center not the airport

We had a miserable connection to Dublin (and were staying in Drogheda, btw). I was so wiped out that I could not even think about doing anything other than eating (definitely), walking (maybe) and reaching the hotel ASAP.

I second the motion to check the luggage somewhere until you think you can reasonably pick up the car and drive to your hotel. It'll help keep you awake, and hey - Dublin! It's walkable and cool!

Near the hill of Tara is Trim Castle. Jet lag will be a problem after a few hours, so upon picking up the rental, start working your way towards your destination. That way you’ll be close to it when you realize you don’t belong on the road any longer. A nice order to visit would be Trim castle, hill of Tara, Slane, then Monasterboice. By then you should be able to check into your cottage.

Thank you all for the great recommendations for the luggage storage. I think we will stow our bags at the airport, take public transportation to the city center and have lunch and do some exploring. I think I'm pretty much set on reserving the car to be picked up from the airport as opposed to driving from a city center rental location. I know the airport will be no picknick but maybe slightly less overwhelming than Dublin. Plus, it looks like enterprise is now operating the Dan Doolet locations in Dublin city center. I know they are the same company but Dooley will work for us. Our secondary driver is under 25, and Dooley seems to be the only car rental that allows us to purchase excess insurance for a younger driver.

If the weather is nice why not enjoy a picnic lunch in St Stephen's Green.

A picnic on St Stephens Green sounds like a lovely idea if the weather cooperates :)