[4] Felim attempted to maintain both a loyal and personal relationship with Henry III King of England, hoping he would limit the influence of de Burgh and other powerful Anglo-Norman magnates in Connacht, but this policy of appeasement produced few concrete results.

In that year Richard Mór de Burgh lead a hosting into the province forcing its then king Aedh mac Ruaidri Ó Conchobair into exile in Tir Eoghain, and bestowing the kingship on Felim.

[14] In the following year William made peace with the exiled Aedh, who resumed his kingship, and afterwards released Felim who went on to make a grant of Kilmore to the Augustinian cleric Cond Craidbheach O Flannacain.

[19][20][21] Felim returned to Connacht gathered all his movable wealth and followers and engaged in scorched earth tactics while retreating further north to seek refuge in the Kingdom of Tir Chonaill.

[21][20] By 1236 this tenuous peace broke down and Felim again sought refuge in Tir Chonaill while Maurice installed Brian son of Toirdhealbhach Ó Conchobhair as ruler of the Irish in Connacht.

[22] De Burgh, who had been in England, rose on behalf of Felim and he was invited from exile by the chiefs O'Floinn, O'Cellaigh and the sons of Aedh Ua Conchobair to regain his rule.

Brian's army consisted heavily of the justicar's troops, possibly as mercenaries,[24] who were routed soon after the start of the battle when Felim ordered an immediate headlong charge at their lines that took them by surprise.

[25] Brian fled the field and Felim imposed his authority on Connacht, putting a fleet on Loch Ce and deposing Cormac King of Magh Luirg.

[21] In 1240 Felim made an unprecedented move for a reigning Gaelic Irish king and traveled to the court of Henry III where he was received, according to the Annals of Ulster, with great honor.

[29] He also traveled to Wales in the same year with a contingent of perhaps 3000 infantry to serve Henry III in his campaign against Dafydd ap Llywelen Prince of Gwynedd and was again pleased with his reception by the English king.

[44] ''Felim, son of Cathal Crovderg O'Conor, the defender and supporter of his own province, and of his friends on every side; the expeller and plunderer of his foes, —a man full of hospitality, prowess, and renown; the exalter of the clerical orders and men of science; a worthy materies of a King of Ireland for his nobility, personal shape, heroism, wisdom, clemency, and truth, died, after the victory of Extreme Unction and penance, in the monastery of the Dominican Friars, at Roscommon, which he himself had granted to God and that order.