The largest building at Clonmacnoise, the cathedral was originally built in AD 909, but was significantly altered and remodelled over the centuries. Its most interesting feature is the intricate 15th-century Gothic doorway with carvings of Sts Francis, Patrick and Dominic. A whisper carries from one side of the door to the other – a feature believed to have been used by lepers to confess their sins without infecting the priests.
The last High Kings of Tara – Turlough Mór O'Connor (died 1156) and his son Ruairí (Rory; died 1198) – are said to be buried near the altar.