The Book of Lismore, containing the written record of the story of "The Siege of Knocklong", was composed around 1480.
It was discovered hidden in the walls of Lismore Castle, Co. Waterford in 1814.

Approximately in the year 250 ADE, midway through his reign as High King of Ireland, Cormac Mac Airt leads his army into Munster to collect extra taxes and tributes.
Cormac is warned by his Druids that his claims are unjust and that his army will be slaughtered. He ignores all advice.

The Siege of Knocklong describes the journey of Cormac and his army southwards. They travel 5 days and 5 nights, arriving on the Hill of Knocklong on the sixth morning of travel. They lose a week enroute due to Druid mischief.

Cormac sets up his camp at Droim Dámhgháire (Knocklong) and Fiacha sets up at Ceann Chláire (Glenbrohane).

Fiacha Moilleathan, King of Munster, and the Munstermen resist Cormac's claims.

The siege lasts over a year and includes five battles between the opposing armies. When the battles fail to bring the Munstermen to submission, Cormac's druids dry up all sources of water in the area, and the siege begins.
This act nearly drove the Munstermen to death. Fiacha turned to the powerful Munster druid Mug Ruith for aid. He restored the water. The Munstermen regain their strength and with Mogh Ruith's help, they defeat Cormac and drive his army out of Munster.

Fiachu and the Munstermen defeat Cormac and his Northern army at Knocklong.