Eoghan Ruadh O Neill, born in 1585, was a descendant of the old O Neill royalty of Tyrone. As a boy he watched the Nine Years War end in Irish defeat and the Treaty of Mellifont in 1603. Like many young Irishmen of his generation he took the Flight of the Earls route into European service, joining an Irish regiment attached to the Spanish military and spending three decades fighting in Flanders during the Eighty Years War. He earned a reputation as one of the best Irish officers in continental service, particularly during the defence of Arras against the French in 1640. When the Irish Rebellion broke out in October 1641 he watched it from the continent and corresponded with fellow exiles, planning his return. In July 1642 he landed at Doe Castle in Donegal with arms, ammunition, and 200 veteran officers and sergeants from the Spanish service. Within two months the Irish Catholic Confederation had appointed him commander of their Ulster Army.

What O Neill found in Ulster was not an army. His predecessor, Sir Felim O Neill of Kinard, had taken poorly trained levies into the field with disastrous results. The Confederate forces in the north were a corps of raw recruits, badly armed, badly fed, and outclassed by every professional unit they had met. O Neill chose Charlemont Fort as his base of operations and spent the autumn and winter of 1642 to 1643 trying to drill them into a real army. He was being squeezed from multiple directions. A Scottish Covenanter expeditionary force under General Robert Monro, headquartered at Carrickfergus, conducted four military campaigns against Charlemont. The Laggan Army, an Ulster Scots militia from the staunchly Protestant district of east Donegal, attacked from the west. By spring 1643 O Neill judged that he had to move his force south into Confederate-controlled territory and let them mature in safer surroundings.

The Laggan Army was the creation of the Stewart brothers, Sir William and Sir Robert, wealthy Protestant landowners in northwest Ulster who had received large grants of escheated land during the Plantation. Both brothers had served the English Crown militarily. After the 1641 rising, in which thousands of Protestant settlers had been driven from their homes, the Stewarts raised a regiment of 1,000 foot and a troop of horse with King Charles's authorisation, originally to defend The Laggan in east Donegal and the adjacent parts of Tyrone and Londonderry. William Stewart had nominal seniority, but Robert Stewart, with his Thirty Years War experience on the continent, took over operational command. Robert was a meticulous recruiter and trainer, and his troops were noticeably better equipped than O Neill's, particularly in muskets. The Laggan Army would become the most effective Royalist militia in the province.

On 9 June 1643, O Neill sent messengers to his outlying commanders telling them to rendezvous with him at Clones. He arrived first, with a convoy estimated at 3,100 individuals: 1,600 fighting men, plus their families, supply wagons, baggage carts, livestock, and the nomadic herders known as creaghts with their cattle. Stewart got the intelligence and marched to intercept with around 3,000 men, including substantial cavalry. O Neill pushed south from Clones towards Scotshouse and reached the Finn River, where Cumber Bridge now stands. He wanted to cross the river before Stewart could reach him. His council of war overruled him. They decided to stand and fight on the north bank, on a stretch of stone roadway bordered by bogs. O Neill placed 100 musketeers at the narrowest point and arranged the bulk of his infantry along the road behind them. He led a cavalry detachment north to slow Stewart's advance.

The cavalry scouting ran straight into Stewart's vanguard. After a short clash O Neill fell back to his army. Stewart sent his own cavalry forward against the musketeers, drove them off, and pulled back to coordinate a full attack. Before that attack could be mounted, O Neill sent his cavalry out to meet Stewart's, and for a while the cavalry battle hung in stalemate. Then an Irish brigadier, Shane Og O Neill, in violation of orders, led his infantry brigade out of its defensive position to attack the Laggan infantry while the cavalry were locked. He never reached his target. The Laggan cavalry wheeled, charged the exposed Irish infantry, and broke them. Half an hour of fighting was enough. As Confederate infantrymen fled south down the road, they collided with the rest of their own army, panic spread, and the entire force dissolved. O Neill organised a fighting retreat, but many of his men ran wherever they could. Irish losses were around 150, including many of the irreplaceable veteran officers from Flanders. Laggan losses were six killed and twenty-three injured. Stewart could not pursue O Neill any further into Confederate territory without leaving his Donegal base unprotected, so he gathered the spoils and went home. O Neill had survived, but his army's training had been set back by months, and the province of Ulster lay completely in Royalist and Covenanter hands.