@Malkynn, are you staying with friends or family? Or in the City Center? Or looking to choose?
One assumption *not* to make about Dublin is transportation. By American standards its fine to good, by European standards it's scant. In particular, there is no train link from the airport. Rather, there are two city bus routes (747 and 757) for €7.50, or more-expensive coaches that drop off around the city.
https://www.dublinairport.com/to-from-the-airport/by-bus/dublin-busesBefore we got a car, we used to take the 757 for travelling, as the line ends near the Green LUAS Harcourt stop. But the 747 is much more frequent. They both pick up at many places in the city center.
The Train lines are the LUAS (light rail / street tram, in the city center) and DART (commuter train, goes along the coast). The LUAS has its own real-time reporting app. You can get discount fares to the trains and buses with a LEAP card. A visitor LEAP card is available at the convenience store on exit from baggage claim (T2: SPAR - most likely from the US; T1 - WH Smith) or you can order one online.
https://about.leapcard.ie/leap-visitor-cardIf you forget to pick one up at the airport, just about any convenience store sells "regular" LEAP cards: €5.00 charge for the card, which is rechargable at any train station, or also through the LEAP card app.
There are no ride-sharing services in the city, but you can summon a Taxi with the Free Now app. There are also a number of taxi queues throughout the city center, including at the South end of Grafton Street / Stephen's Green.
Dublin is very much a walking city. I am 6' 8", and a former distance runner. I walk very comfortably along with the commuter walking traffic. This can be quite disconcerting for tourists trying to get along, particularly if they are also consulting a map.
There are several hop on / hop off buses. They are partially open on top, which could be brutal in December. But they will give you a quick orientation to things, and can help connect more distant sights, like the Guiness brewery.
As a boom town, any mode of transportation is Dublin is a nightmare around commuting times. The trains are packed like Tokyo from 8-9 am, and again 4:#0-5:30-ish (worse in the morning) There is a thorough bus system, but in the high traffic it's more about saving you walking distance than time. (the 20 minute walk which finished my commute is a 15 minute drive in morning rush hour) On holidays or outside of commuting, it's fine.
I agree with Dave's comments. I particularly love the Long Room at Trinity College. That's the building where the Book of Kells is at, but there are so many more artifacts there: a copy of the Irish Declaration of Independence, Brian Boru's harp ("the" harp of Ireland) many original literary manuscripts. And the room itself is gorgeous, if you are a library person.
Time for a meeting--let me know if you want more of this, or getting on to more sights...