Sean O'Hegarty commanded the IRA's 1st Cork Brigade, and the flying column at Coolavokig drew its men from the 1st, 7th, and 8th battalions. Dan Sandow O'Donovan and Dan Corkery were his principal officers. They had sixty rifles, two Lewis machine guns, a handful of shotguns and revolvers, and significantly no grenades. The men were divided into four sections covering different fields of fire above the road. They had occupied the ambush position over several days, and word had reached the British that something was happening west of Macroom. The Auxiliaries came forward armed for it: seventy men, plus seven RIC constables, plus rifles and machine guns and grenades of their own, carrying four Irish hostages in the convoy as insurance against attack.

The plan depended on letting the lorries enter the killing zone before opening fire. An IRA volunteer left his position briefly and turned back too obviously, and the leading lorry crew spotted him. The British column was already on alert; now it was forewarned. They began approaching with caution, but the road geometry forced them in, and when half the lorries had entered the trap O'Hegarty's no.1 section opened fire with rifles and a Lewis gun. Ten rifles from no.4 section joined them. According to the Irish military journal An Cosantoir, one British lorry immediately turned around and sped back to Macroom. The shooting was point-blank for the lorries closest to the ambush, and the Auxiliaries had to fight from exposed positions while taking cover behind the vehicles.

Major James Seafield-Grant commanded the Auxiliary convoy. He was at the front of the column when the firing started. As his men took casualties and ground gave way, he moved among them trying to rally their position. He was killed while doing so. The British retreated into two nearby cottages, dragging wounded with them. The IRA's no.2 and no.3 sections had been swinging east along a hillside trying to encircle the British northern flank but could not close inside 500 yards. The volunteers near the road closed in on the cottages, preparing to assault them. They had no grenades; they intended to set the buildings on fire. Before they could act, large numbers of RIC reinforcements appeared from the east and began encircling them. After a half-hour rear-guard action, O'Hegarty's column broke off and retreated northwest into the hills, just as soldiers of the Manchester Regiment arrived to finish what the Auxiliaries had started.

The IRA suffered no casualties. The British casualty count was disputed at the time and is still disputed. British authorities reported that Major Seafield-Grant was the only Auxiliary killed during the engagement itself, with two more dying later of wounds. The IRA claimed between fourteen and sixteen British dead. Most modern accounts settle on ten Auxiliaries killed, including the Major, with others wounded. What is undisputed is what happened afterward. It later transpired that the Auxiliary force at Coolavokig had been part of a much larger sweep planned for that morning, with around 600 British Army troops mobilising from Cork, Ballincollig, Bandon, Clonakilty, Skibbereen, Bantry, Dunmanway, Millstreet, Macroom and Killarney. The sweep was meant to crush the IRA in mid-Cork. Instead, the column it was meant to encircle ambushed its leading element.

British forces stopped raiding and patrolling the area west of Macroom. They were reluctant to enter the hills around Coolavokig and Ballyvourney at all, and when they finally returned it was with a force of 2,000 men. The Manchester Regiment recorded in its Digest of Service that on 18 March 1921, three weeks after the ambush, a party proceeded to Coolavokig to destroy houses believed to have been used by the IRA. That was a common reprisal, and the cottages where the Auxiliaries had taken cover during the battle would have been an obvious target. The men who died that morning were soldiers and policemen on both sides, with families and futures of their own. Major Seafield-Grant was a career officer doing his duty. The young volunteers in O'Hegarty's column included farmers' sons from townlands all over mid-Cork. None of them had asked to be in a guerrilla war in February in the rain. By the time of the truce in July, the road they had fought over was effectively in IRA hands.