The Ciannachta tribe are the main focus of Cath Crinna. The myth mightve been rewritten by Cormac mac Cuileann frim an earlier ulster story to show that his dynasty and not the Ui Neill were responsible for the Ciannachtas contemporary territories in ulster and leinster.
Cormac was constantly at war with the Ui Neill and he mightve wanted to woo the Ciannachta over to his side. So in Cath Crinna he has a mythological munster over king rallying his sub tribes for a battle. The founder of the Ciannachta Teigue agrees to fight with the Eoghannacht because theyve a common ethnicity… and so theyre granted lands by the Eoghannacht.
The narrative of myths is always fiction but this fiction presaged Cormac mac Cuilleanns death. He died in battle against the leinster Ui Neill and never managed to woo the Ciannachta.
In the end he started openly trashing Leinster and his dynasty went down in flames so badly the Dal gCais were the next munster tribe to challenge the high kingship. They were nobodies who took advantage of the power vaccuum Cormac left behind him.They grew into the Ui Briain dynasty under the uber famous Brian Boru.