Not too much I can offer, as whilst I was working there for a while, it was a weekly commute from England, so I saw a fair bit of Dublin, but not much else.
The car is the way to go and whilst you could take the train from Dublin to somewhere else to then hire a car, I'm guessing you'll fly out of Dublin as well, so a round trip will make more sense... even if only to know for sure where the car needs to be dropped off.
If you do spend time in Dublin itself, then:
- Rush hour traffic is quite bad, so try to avoid 8-10am / 4-6pm
- Against the Grain is a cracking real ale pub, with a massive range of national and international beers.
- The standard of beer / stout is high.
- The standard of food impressed (and surprised) as well, and there are also bargains to be had via early evening deals (say 5:30-7:30pm)
- Generally, avoid the Temple Bar area. They're rather touristy pubs and beer touristy can sometimes get messy.
One final thought - don't discount Northern Ireland, and the border should still be pretty seamless.