Also near Dublin is Newgrange, the impressive neolithic passage tomb. They don't take reservations, but entries are limited, so unless you show up very early, the tours might be sold out for the day. You can still see it at a certain distance, and visit the exhibit. Newgrange is older than Stonehenge, and older than the pyramid.

Also very interesting, for me at least, is the Book of Kells, and other ancient manuscripts, in the Trinity College library.
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