Complex sentence: The place she was buried was called Carman after her, and the Tuatha Dé Danann are said to have instituted a festival there, the "Óenach Carman", which was celebrated in historical times.
Split into simpler sentences: The place she was buried was called Carman after her, and the Tuatha Dé Danann are said to have instituted a Óenach, or festival. The "Óenach Carman", which was celebrated in historical times.