The geography did the planners' work. Narrow Water Castle sits on the northern bank of the Newry River, the tidal channel that here marks the border between Northern Ireland and the Republic. On the southern shore, the wooded Cooley Peninsula in County Louth rises directly from the water. A bomb crew positioned there had cover, escape, and a border the British Army could not cross. Soldiers knew the danger of this stretch and often declared the road out of bounds, but using it occasionally was meant to avoid a predictable pattern. On that summer afternoon, two lorries and a Land Rover of the 2nd Battalion of the Parachute Regiment rolled past Narrow Water Castle, and the pattern caught up with them anyway.

The first explosion at 16:40 destroyed the rear lorry and killed six paratroopers. The driver, Anthony Wood, was twenty-five years old. Only his pelvis remained in the wreckage, welded to the seat by the heat. Two soldiers in the same vehicle survived with serious injuries. Surviving paratroopers radioed urgently for help and set up what they had been trained to set up: an incident command point, sheltered behind the stone gateway across the road. Reinforcements began to arrive by road, including senior officers. A Wessex helicopter came in to evacuate the wounded. The pattern of British response after a bomb was a well-studied thing. The South Armagh Brigade of the Provisional IRA had studied it carefully.

At 17:12, a second 800-pound bomb hidden in milk pails at the stone gateway detonated as the helicopter was lifting off with the wounded. It struck the command point exactly as the IRA cell on the far bank had predicted. Twelve more soldiers were killed, among them Lieutenant Colonel David Blair, the second lieutenant colonel to die in the Troubles. Major Mike Jackson, who would later command the British Army, arrived at the scene shortly afterward and found human remains in the road, in the river, and in the trees. He was asked to identify the face of his friend Major Peter Fursman, recovered from the water by divers from the Royal Engineers. A press photographer at the scene, Peter Molloy, came close to being shot by a paratrooper enraged at finding him taking pictures rather than helping. Molloy said later: "I had trespassed on the worst day of these fellas' lives and taken pictures of it."

In the chaos after the first blast, British soldiers fired across the river into the Republic. They killed William Hudson, a 29-year-old Englishman from London, and wounded his cousin Barry Hudson, a 25-year-old from Dingle in County Kerry. The men were on holiday on the Cooley Peninsula near Omeath. They had no connection to the IRA, the British Army, or the ambush. The Republican Army's first statement on the incident denied that any shots had been fired at the troops at all; the official inquiry left open whether the soldiers had mistaken ammunition cooking off in the burning lorry for hostile fire. What is certain is that the Hudson cousins were tourists at the wrong stretch of riverbank on the wrong afternoon. William Hudson did not come home.

Sixteen of the eighteen British soldiers killed at Warrenpoint were paratroopers. They were members of the 2nd Battalion of the Parachute Regiment and the Queen's Own Highlanders. The youngest was nineteen. Most had been in Northern Ireland only weeks. In republican areas, graffiti appeared in the days that followed reading "13 gone and not forgotten, we got 18 and Mountbatten," a reference to the thirteen civilians shot by paratroopers on Bloody Sunday in Derry seven years earlier. The day after Warrenpoint, the Ulster Volunteer Force shot dead John Patrick Hardy, a 43-year-old Catholic civilian in his Belfast home, in retaliation. He was not an IRA member. Brendan Burns, later named as one of the bombers, was arrested by the Gardaí shortly after the attack and released for lack of evidence; he died in 1988 when a bomb he was handling exploded prematurely. No one was ever convicted of the Warrenpoint killings.

Margaret Thatcher visited the site within days. The British military reorganised its security command in Northern Ireland; Sir Maurice Oldfield was appointed to coordinate intelligence between the Army, MI5, and the Royal Ulster Constabulary, and the RUC was expanded by 1,000 officers. Historians describe Warrenpoint as the moment that hastened the policy of "Ulsterisation," replacing British military patrols with local police where possible. On Bank Holiday Monday each year families of the dead gather at the memorial near Narrow Water Castle, on a stretch of road that is once again quiet, with the green border woods of County Louth a few hundred metres away across the river. Lieutenant Colonel Blair is also remembered on a memorial at Radley College in Oxfordshire, where he had once been a pupil.