Newgrange, a day trip from Dublin, is 5000 years old. The photos kind of suck because it was pouring rain and it wasn't listed in my stupid GPS so it took hours and hours of driving the wrong way until we found it and thus we were a little stressed on arrival. Other than that, an incredible ancient site that was one highlight of our Irish adventure.
There are 3 giant mounds that you can visit (only one you are allowed to enter) that are pitch black inside. They are lit up for 17 minutes every year on December 21st by the sun shining through a little window. If you want to go there on the winter solstice, you can get on a list for a lottery where they choose 50 lucky people with a loved one to go there for the main event. If it is cloudy, too bad, you don't get to see it. Last year over 30,000 people were on the list.
The carved stones that surround the mounds were moved there from 80 km away. This is before the wheel was invented. They estimated that it took 60 years to build the mound, which is 3 generations of people working to make this burial site super special.