Flying into Dublin and renting car 2 days later

TravelingTea-jump on Google Maps and take a look. For every B & B or hostel or hotel, pop the address in and you'll know. There are lots of rentals and hotels far outside the city center, and looking at the map is the only way to be sure of location and convenience.
We stayed at one of the B & B's on Drumcondra Road Upper-find that on the map-and had numerous bus stops right outside the door to travel the 2-3 km to the heart of Dublin, or to Kilmainham Gaol, for example.
Budget car rental's city office is near there too; we picked up our car there on our last morning in Dublin and headed out of town, away from the city center. We were going west, and didn't want to pick up at the airport. We immediately got lost, and you probably will, too, but that's another story.
Look at a map and learn where things are that you want to see, and enjoy Ireland. Buy a Michelin map for your driving while there.

Best, Dave

Take the Airporter bus into the city from the airport and buy a roundtrip ticket and use it on the way back. It's 15 to 20 minutes into the city depending on where you stay. Dublin is pretty small so staying anywhere near the city center is probably the same - you'll end up walking to all the various places anyway. If you want to check the airport bus route and make sure you're near the line it's a decent way to go about it but I wouldn't take a close BnB over a recommended one that is a little further away.
We stayed on the "far" side of the river so we had to cross the bridge to get to most of the sights but even that wasn't that big of an inconvenience.

Have a great trip,
=Tod