If you are interested in the history of humanity, you have to go to Bru Na Boinne – 45 km away from Dublin.
I will not bore you with what this place is, the Internet has enough information if you are interested. I will just give you some practical details.
First, park at the Visitor Center, it’s free.
Once you will tell that you want to visit (I highly recommend both Knowth and Newgrange) you will receive two stickers with the time to visit each of the sites. The ticket costs 13 euro for both. Dowth is completely destroyed and is not even visible anymore as I understand and is used nowadays by archeologists.
Knowth is the biggest tomb, but you are not allowed to go inside. Newgrange is a bit smaller and you can see the architecture of the rocks that make the entire “building” completely waterproof since thousand of years. I recommend visiting both, but if your time is really limited, go for Newgrange.
Knowth:
Newgrange:
The map of the area:
The path to the visitor center:
If you are lucky, the time to visit both of them is 3 hours. I had to wait in between 30 minutes, so it took 3h30″. But don’t worry, there’s a nice restaurant inside the visitor center, so you can pick a quick coffee or eat for lunch.