Christ Church Cathedral is an amazing architectural treasure, a beautiful monument to medieval Dublin and Irish history generally. Taking a tour of the catacombs is an interesting experience, although not the best of the tours available in Dublin. It is pretty cold and drafty in the basement of the church, and the tour tends to drag on a bit with recounting of various English lords and Catholic bishops who were buried in the tombs below. It's worth a visit if you go to Dublinia first and then want to take a quick peek around the inside of the cathedral - but the tour is not going to rank highly on your list of things you did in Ireland.
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