ventually Fiacha Muilleathan, son of Eoghan Mor, assumed the kingship of the two Munsters and during his reign Cormac Ua Cuind, king of Ireland, came (from Tara) with a hosting into Munster, demanding tribute from the two provinces of Munster. Cormac besieged the Munstermen at Druim Damhaire (Knockloag); this king weilded great power, both by the vastness of his army and the power of his druids. Cormac had British druids weaving their spells against the Munstermen, so that by necromancy they had dried up all the wells and rivers of Munster, so that the people and their lands were in danger of death from the want of water.
Then Fiacha Muilleathan sent for Mogh Ruith son of Fergus, the best druid to be found in Ireland. Mogh Ruith then lived in Oilen Dairbhre (Valentia Island) in his old age, blind and decrepit, as he had outlived nineteen kings of Ireland:- from the time of Roth mac Rioghuill (the druid who had trained Mogh Ruith in sorcery) to the time of Cairbre Lifechair son of Cormac mac Airt. Mogh Ruith then came to meet Fiacha Muilleathan and the Munster nobles and they complained about what the druids of Leath Cuinn (Northern half of Ireland) had done to them. He undertook to oppose their magic spells, and he chose the territory of Fir Mhwige (Fermoy) as his reward. Mogh Ruith then overcame the druidery of Cormac and they defeated Cormac's forces routing them from Knocklong to Tara with a great massacre. Fiacha Muilleathan the Munster king did not leave Leath Cuind until he got hostages and homage from Cormac mac Airt; as the poet Feidhlime mac Crimthann wrote:-
Good was the king Fiacha Muilleathan
A great territory the Half over which he ruled
He brought hostages from Tara the Strong
To Rathfuim to Rath Naoi
Though he was great; Cormac Ua Cuinn
He bowed to the king of Tir Duinn (Munster).
Fiacha Muilleathan had three noble sons, Oilill Flann Mor and Oilill Flann Beag and Deachluath. The latter, Deachluath is ancestor of the tribe called Uí Fiachach Eile (in north-east of Tipperary - Thurles and Roscrea) and Oilill Flann Mor left no issue.
The family of Oilill Flann Beag.
Oilill Flann Beag had four sons, i.e.,
1. Lughaid, ancestor of all the Eoghanachta;
2. Fiodach, father of Crimthann;
3. Daire Cearba from whom was the Uí Liathain;
4. Maine Munchaoin from whom the Uí Fidgeinte; (the O'Donovans and O'Collinses of mid and west Limerick)
Fiodach, to him was son Criomthann Mor mac Fiodhaig from whom is the tribe Clann Crimthann. He was king of Ireland and Britain.
It is this Crimthann who took the fortress of Doire Da Broc from the sons of Eochaid Mugmeadhon (of the kings of Tara) ie., from Brian, Fiachra, Oilill and Feargus. Crimthann's sister Muingfionn was mother of those four sons. So that her son, Brian, would get the kingship of Ireland, she plotted to poison her brother, Criomthann; the latter died of that poisonous drink; and Mungfionn herself died as well at Inish Donglais on the Moy (Co. Mayo) - as she tasted the drink in order to induce her brother to drink from it. Crimthann having drunk it came to Sliabh Uidhe on Riogh "The Mountain of the King's Death" (now Cratloe Hill, Co. Clare) and there expired.
The four above names sons of Oilill Flann Beag divided Munster between them as follows.
From Comar no dTre in Utsge (Waterford Harbour) to Belach Conglais (Cork Harbour) -
Luguid ancestor of the eoghanachta and Fiodach father of Criomthann took this half;
from Belach Conglais (Cork Harbour) to Limerick -
Daire Cearba and Maine Muncharim to this other half.
These latter two Daire and Maine were born at one birth and concerning them (before birth) their mother saw this vision: they were back to back in her womb and a black chafer between them. This was interpreted as follows: The tribe of Mogh Ruith the druid was the chafer placed between the Uí Liathain and the Uí Fidgeinte so that neither could come to the help of the other.
Sources : Eugene O'Keeffe