In 1250, taking advantage of a dispute about the succession, FitzGerald invaded Tir Conaill but did not remain there. In 1252, he renewed the attack, building a new castle near Belleek and another on the eastern frontier near Banbridge. The viceroy also came on with a strong army, penetrating into Tir Eoghain by way of Armagh. O'Neill bent before the storm and made submission. This was the culminating point. Next year, 1253, hoping to enforce his advantage, the viceroy once more invaded Tir Eoghain, but this time he obtained no submission and was forced to retreat with heavy loss. O'Neill forthwith took the offensive, invaded the Earl of Ulster's territory, and destroyed a number of castles including the new castle near Banbridge. There is a lull at the turning of the tide. For several years, hostilities cease on both sides. Then in 1257, Godfrey O'Domhnaill, king of Tír Conaill, destroys again the castle of Caoluisce near Belleek and attacks Sligo, burning the town. Retiring, he fights a rearguard action, and both he and Maurice FitzGerald receive wounds of which they afterwards die. Under the following year, 1258, is chronicled an event in itself of the greatest significance and also an index of the significance of foregoing events. Of the unsubdued Irish outside of Ulster, the chief potentates at this time were Tadhg O'Briain, king of Thomond, and Aodh O'Connor, king of Connacht, son of Feidhlimidh who had cast off the authority of FitzGerald and De Burgh. These two kings assembled their nobles and their forces and marched together to Caoluisce on the Erne, the site of the demolished fort. They met there Brian O'Neill, king of Tyrone, "and," says the annalist, "all those nobles gave the supreme authority to Brian O'Neill." That is to say, so far as lay in their power, by a spontaneous act, they restored the monarchy of Ireland. Therefore, when I say that Brian O'Neill's defence of Ulster, with the co-operation of the kings of Tir Conaill, marks the definite rallying point against the Norman conquest, I give something more than a private opinion or a modern inference. It is a fact to which, in the year 1258 on the banks of the Erne, the kings and nobles and fighting men of Thomond and Connacht, as well as of Tyrone, render the clearest and most solemn testimony possible. Never before in Irish history had the chief provincial kings thus spontaneously and peacefully awarded the high-kingship to one of their number. The act implied a repudiation of the authority that set up feudal lords over Irish kings, and amounted to a declaration of national independence. Half a century later, Brian O'Neill's son, in a letter to the Pope, again declares the Plantagenet lordship of Ireland to be null and void and asserts the right of the Irish to determine their own sovereignty.