The name Castlelyons comes from Caislean Ui Liathain - the castle of the Liathain.
The Ui Liathain was a family related to the Eoghanacht who ruled Munster. Their fortress is thought to have been on a site near the present castle. The name Ui Liathain comes from the sons of Eochai Liathain who lived in the 5th century A.D. He was the grandfather of Mac Coille who gave his name to Ui Mac Coille or what we now know as the district of Imokilly.
The Ui Liathain had invaded East Cork supplanting a pre-Celtic people known as the Tuatha Cairge and the Erna. From the 5th to the 11th centuries they ruled over East Cork from Corrin to the sea at Youghal and Cork
Some interesting links: Much of the information in the "History
of Castlelyons" included in this site was researched by Fr. David O'Riordan
(former CC of Castlelyons) and taken from his book "Castlelyons".
Census of Castlelyons in 1911
Slater's National Commercial Directory of Ireland
1846
Cork Ancestors
The Barony of Barrymore
William Brouncker - Mathematician
Peadar Ua Laoghaire