Dublin is fabulous..... I just loved it when we had a couple of days there some years ago. It is full of quirky stuff ( well this is Ireland....

) like statues {Molly Malone, the reclining nude known as 'the Floozie in the Jacuzzi', the two gossiping women known as 'the Hags with the Bags', James Joyce ('the Prick with the Stick'), Oscar Wilde (in Merrion Square Park)}, some gorgeous buildings - and of course Guinness. A visit to the Guinness Brewery is a must, not only is it fascinating, but the admission price includes a pint of the real stuff, which you claim in the bar at the end of the tour. Unfortunately, it broke my heart to see how many rejected pints were passed back when visitors who had come out of curiosity discovered that they didn't really like the stuff after all.....

. It is well known that in fact Guinness doesn't travel very well, so the best place to buy it is close to a brewery - and what could be better than this. It is truly like drinking liquid velvet....

. However, I have to say that even so, the very best pint I enjoyed in Dublin was not there, but in a pub a short distance away - nothing special (I can't even remember the name) but just the most perfect Guinness ever
As for Gogarty's - yes it is fantastic, but hopelessly overpriced (they make all their money from the tourists and you won't find many natives in there!) . One of the theories is that the price goes up on the hour every hour - so make sure you order your drinks at 5 minutes to....... This tends to apply to any venues offering 'authentic' Irish music, however, so try shopping around.
The other 'must' in Dublin is the Book of Kells at Trinity College, well worth seeing. In December you are not likely to come against the queues that build up in the height of the season, which will give you time to look at it properly. There are a lot of other interesting things on view there as well.
As always, I totally recommend the "Rough Guide" for advice on practicalities as well as what to visit. We have found it worthwhile to try borrowing this from a library if you don't want to buy it outright - although the investment gives you a nice souvenir to remind you of your visit.
I should perhaps declare an interest, as I have long had a fascination with Ireland because my birthday is on St Patrick's Day. I am ashamed that I have only managed one visit to the Republic (a fantastic week, including these two days in Dublin) and one to the North (several days in Belfast, with wonderful trips to the Mourne Mountains and up the unforgettable Antrim Coast.
Hope you have a great time.