Christ Church Cathedral predates every other standing structure in Dublin. The original wooden church was founded around 1028 by Sitriuc Silkenbeard, the Norse King of Dublin; the stone building you see today was begun by the Anglo-Norman Richard de Clare (Strongbow) after his conquest of Dublin in 1169.
The interior is impressive for its sheer age. The tomb traditionally identified as Strongbow's — actually a replacement from the 1200s — lies in the nave. The Romanesque south transept doorway, the medieval tiles, and the unusually wide nave all reward close attention. The magnificent crypt, stretching the full length of the building, is the largest and oldest such structure in Ireland or Britain.
The cathedral is connected by a covered stone bridge to the Dublinia museum next door, which recreates Viking and medieval Dublin. A joint ticket covers both.