I'm extremely jealous of all of you going. To give a bit of background, I was born and raised in Ireland, and moved from Ireland to Fort Worth when I finished university in 2009. I went to every home game during the 2009 and 2010 seasons, and then moved to NYC about a week after we won the Rose Bowl in January 2011.
I'm now a US citizen with an American wife and two American kids just outside NYC, but I've been a committed Frogs fan for the past 17 years.
We had a wedding back in Ireland last month, and I can't financially justify going back again for the game a month later.
I'm from Donegal, which is the county in the very northwest about a four hour drive northwest of Dublin. It is absolutely beautiful up there, but depending on how short your trips are, it might make sense to stay in the Dublin area. If you have a longer trip, I would look into it. Mountains, beaches, great pubs, traditional Irish music, and golf. (My local course growing up was Cruit Island - look up the 6th hole on YouTube)
I would definitely recommend the Gravediggers pub for a Guinness and a coddle (an Dublin dish that looks awful, but tastes great), and also get a guided tour of Glasnevin Cemetery right next to it. I did that this past October and it was fantastic.
On GameDay, I would recommend The Palace Bar in Dublin city center. It is one of *the* go-to pubs on gaelic football and hurling gamedays. It's closer to Croke Park (the GAA stadium) than it is to Aviva Stadium (the soccer and rugby stadium, where the TCU game will be), so you would probably need to get taxis over there closer to game time. Bowe's Lounge Bar is also close to there and is a good spot for Guinness.
Slattery’s and Beggar’s Bush pubs are closer to the Aviva and are supposed to be good gameday spots. I've never personally been to those, but have heard good things.
If you are under 30 years old, I would recommend Copper's nightclub (Copper Face Jacks). You know how most college towns have a [ Finebaum ]ty bar or nightclub where EVERYONE goes? This is Dublin's version. If you are older than 30, I would avoid, as it's probably drunker and younger than you need it to be!
Food wise, it is miles better than it was when I emigrated, so there are a bunch of good options, but Bambino pizza gets rave reviews from everyone who goes there, including New Yorkers who can be snobby about pizza.
For those using rental car insurance covered by your credit card (I know Chase Sapphire has an auto-rental policy built in), call your credit card company/bank, and tell them that you are panning to use the card to cover your rental car insurance in Ireland, and to send/email you a letter confirming your eligibility waiver. I have landed there before, and Ireland is one of a couple of countries where the rental company will sometimes not accept that without proof of coverage. And sometimes your American phone will have trouble connecting to the internet in the Dublin Airport car rental office, so print out a copy (or a few) of the eligibility waiver letter so that you're not forced to take out additional, surcharged insurance that you don't need.
If anyone is planning to travel to the UK (including Northern Ireland), you will need to apply for an Electronic Travel Authorization visa waiver. It only about £10 to £20 per application, but they will check it at British entry points. You can drive to/through Northern Ireland and not have to present it, but if you were in a situation where you had to present it (like if you had a car accident) and you didn't have it, it would cause issues.
Anyway, that's a long stream of consciousness that I hope will be a bit helpful. GO FROGS!
(I have major FOMO)