That same night, IRA Captain Martin Forde of Bekan Company led a unit of 25 armed and masked men in stopping a train running from Ballyhaunis to Westport. The cargo included steel shutters intended for the Royal Irish Constabulary barracks in Westport - part of a country-wide programme to fortify RIC stations against exactly the kind of attack the IRA was making on them. The volunteers removed the shutters and buried them in the nearby bog. A useful denial; better still, the same men then moved across to Holywell Wood, where the rest of the local IRA were gathering. Word had spread fast: the bogged-down petrol lorry, lightly guarded, was an opportunity worth taking.

About forty IRA volunteers assembled and marched on the British camp. Their first problem was that they could only see one sentry, and they did not know how many soldiers lay beyond him. An IRA officer simply walked up to the sentry and asked for a light for his cigarette. The sentry sent him into the camp. As he stooped to take a light from the campfire, he counted: eighteen rifles stacked nearby. Eighteen rifles meant roughly eighteen soldiers. He thanked the men, walked back into the dark, and reported. Commandant Patrick Kenny made a plan. But as his men crawled through the fields toward their positions, motorcar headlights appeared on the Claremorris road. The exposure was too dangerous. Kenny postponed the attack to the following night.

There was a sports day in Aghamore that Sunday and many of the battalion's volunteers were easily located. By nightfall the IRA had mobilised everyone available - around 188 men, many of them unarmed, set to scout the surrounding roads, dig a trench across the road to delay British reinforcements, and form outpost cordons. Members of Cumann na mBan, the women's republican organisation, ferried ammunition from arms dumps to the ambush site through the day. D Company from Brackloon took up positions about half a mile down the road. The plan was now refined: while the soldiers slept, Kenny would lead about twenty men - armed with shotguns and revolvers - into the camp and try to take the rifles before the Highlanders could react. At about 3 a.m. the small ambush party crept forward.

The attempt to take the weapons did not work as planned. The Highlanders woke. Kenny's men managed to put three of them out of action immediately, but the rest organised a defence and returned fire. The IRA party retreated to positions behind a fence and kept up steady fire on the camp. Accounts of the duration vary - some say fifteen minutes, others one to two hours. By dawn the engagement was over. The IRA had failed to seize the rifles, which had been their primary objective. Commandant Kenny was badly wounded, the only IRA casualty of the night despite varied claims by both sides. The British admitted three soldiers wounded; the IRA later believed they had inflicted between five and ten casualties. In a country-side war fought largely with shotguns and small arms, the line between success and failure was often a few minutes of timing.

The aftermath says something about how the rural war of independence was fought. Kenny was carried from the field by volunteers Jack and William Caulfield, taken first to a house nearby, then to Pat Healy's house, then to Mayo County Infirmary, where Dr McBride treated him. Within 24 hours it was deemed unsafe for him to stay there, so he was moved to a disused part of the Union hospital in Castlebar, where Cumann na mBan members working in the hospital had established a secret IRA ward for wounded volunteers. After ten days under Dr Hopkins he was moved to Galway for five weeks of treatment under Surgeon M O Maille. Then four months of convalescence at the home of Padraic O Maille TD near Maam in Connemara, a safe house used by three IRA brigades. Kenny returned to Ballyhaunis in April 1921. The British military, meanwhile, raided 150 private houses in the surrounding districts looking for the wounded man and the men who had taken him. They found nothing incriminating.