I highly recommend this book from historian and archaeologist Brian Lacey.

“Cenél nEógain, a royal dynasty from Inishowen in Donegal, defeated its previously more successful opponent, Cenél Conaill – also from Donegal – at the battle of Clóitech in AD 789. All changed following that battle. From then on, the Cenél nEógain kingdom, known as ‘Ailech’, spread across Counties Derry, Tyrone (giving that territory its name), into Armagh, and dominated the whole territory of Ulster.”

Available from Four Courts Press, Dublin

https://www.fourcourtspress.ie/books/2025/cenel-neogain-and-the-donegal-kingdoms-ad-8001200