Conor na Siudaine O'Brien
O'Brien, Conor na Siudaine, King of Munster, succeeded his father in 1242. With twenty other Irish princes, he was summoned by Henry III. to aid him in an expedition against the Scots, and afterwards, the Four Masters record that "a great battle broke out between him and the English of Munster." The territories of all the Irish princes but the O'Neills, the O'Conors, and the O'Briens had long before this been partitioned amongst the descendants of the Norman invaders.
In 1258 a conference was held at Caeluisce (Narrow-water) on the Erne, between Hugh O'Conor and Teige O'Brien, on behalf of their respective fathers, and Brian O'Neill, to concert measures for mutual safety. They made peace with each other, and conferred the, sovereignty of the island upon Brian O'Neill. Little practical result followed this compact; several Irish princes were soon detached from the alliance by the Anglo-Normans, and next year, when O'Neill and O'Conor collected their forces, no representative of the O'Briens joined them.
The battle of Drumdearg, near Downpatrick, ensued, in which the Irish were defeated with the loss of Brian O'Neill, and a large number of Ulster and Connaught chieftains. On the other hand, O'Brien defeated the English at Kilbarron, in Clare, where many of the Welsh settlers of Mayo were slaughtered. He was then strong enough to compel several of the ancient tributaries of his house to acknowledge his authority. He fell at the battle of Siudan, in Clare, in 1267, in an expedition against the O'Loughlins and O'Conors of Corcomroe.
Sources
263. O'Briens, Historical Memoir of the: John O'Donoghue. Dublin, 1860.