The ambush is organized by Major General Michael Kilroy, later Commandant of the 4th Western Battalion of the IRA. He and his flying column of volunteers take up position between Widow Sammon’s House and that of Widow McGreal in Carrowkennedy and await a Royal Irish Constabulary patrol.
At 6:30 PM, a scout signals the approach of the patrol, which includes two Crossley tenders and a Ford motorcar. Jimmy O’Flaherty, a former Connaught Ranger, lines up his sights on District Inspector Edward J. Stevenson in the lead vehicle. Stevenson is killed by a bullet through the centre of his forehead. The lorry lurches forward and stops in the middle of the road and comes under heavy fire from the hillside above. Police tumble out quickly and get down behind a bank which gives them some cover. A Lewis gun is thrown out and trained on the third section of IRA men. After two short bursts of fire, the gunner lay dead beside his gun. A second gunner fires a burst of shots from the Lewis in the direction of the third section, then he swings the muzzle in the air to protect himself from the riflemen above. This is unsuccessful and he also falls dead beneath the gun. Four men in the lorry are now dead and the remaining men are led by Sergeant Creegan. They attach a grenade launcher to a Lee–Enfield rifle and keep the IRA at bay.
The second lorry is stopped by gunfire from both sides of the road as soon as shots are heard from the direction of the first lorry, killing the second driver. This lorry coasts to the ditch at the side of the road. After a while the men run towards McGrale’s thatched cottage facing the road. They poke rifles through the front windows and through a window high in the gable which looks down on the Westport road. They use up a lot of ammunition unnecessarily and then realize that they have left their spare ammunition in the lorry. They unsuccessfully try to persuade the Widow McGrale and her young son to fetch the ammunition.
The motorcar is some distance behind the second lorry and stops beyond the cottage. Three men jump off the exposed side and two remain on the sheltered side of the road which has a thicket beside it next to the cottage. One of the policemen advances towards the rebel position but is badly wounded.
Two hours later, Michael Kilroy is worried that if the first lorry does not surrender soon, the column might not have time to concentrate on the police in the McGrale cottage as enemy reinforcements could arrive at any time. A fresh assault on the lorry is made by Johnny Duffy and Tommy Heavey who have bayonets. A rifle grenade, which is hurled by the police, falls back into the lorry and explodes, killing the man who threw it and fatally wounding other police beside him. A handkerchief of surrender is hoisted on a rifle by Sergeant Creegan, who is fatally wounded in the legs and abdomen.
The captured Lewis gun is fired on the McGreal house from a covered position by O’Flaherty. The men inside come out with their hands above their heads.
The IRA column captures 22 rifles, eight drums for the Lewis gun, several boxes of grenades, 21 revolvers, and approximately 6,000 rounds of rifle ammunition. Petrol is poured over the two lorries and the motorcar and they are set ablaze.
Eight of the British are killed outright or die of their wounds and sixteen surrender. The Black and Tans who surrender are not killed, even though this policy has been endorsed by IRA General Headquarters due to the terror and mayhem they inflict on civilians. Many of the local people go into hiding to avoid the retribution of the Tans. The IRA volunteers escape arrest by sheltering in safe houses.