From Wikepedia:
There is no consensus as to who fired the fatal shot. The most recent authoritative account suggests that the shot was fired by Denis ("Sonny") O'Neill, an Anti-Treaty IRA fighter and a former British Army marksman who died in 1950.[24] This is supported by eyewitness accounts of the participants in the ambush. O'Neill was using dum-dum ammunition, which disintegrates on impact and which left a gaping wound in Collins' skull. He dumped the remaining bullets afterwards for fear of reprisals by Free State troops.[24] Collins' men brought his body back to Cork where it was then shipped to Dublin because it was feared the body might be stolen in an ambush if it were transported by road.[24] His body lay in state for three days in Dublin City Hall where tens of thousands of mourners filed past his coffin to pay their respects. His funeral mass took place at Dublin's Pro Cathedral where a number of foreign and Irish dignitaries were in attendance.


Collins' shooting has provoked many conspiracy theories in Ireland, and even the identity and motives of the assassin are subject to debate. Some Republicans maintain that Collins was killed by a British "plant". Some Pro-Treaty accounts claim that de Valera ordered Collins' assassination. Others allege that he was killed by one of his own soldiers, Jock McPeak, who defected to the Republican side with an armoured car three months after the ambush.[25] However, historian Meda Ryan, who researched the incident exhaustively, concluded that there was no real basis for such theories. "Michael Collins was shot by a Republican, who said [on the night of the ambush], 'I dropped one man'". Liam Deasy, who was in command of the ambush party, said, "we all knew it was Sonny Neill's bullet."[26]

The following is an excerpt from Tom Barry's book 'Guerilla days in Ireland' where he is at pains to scotch the theory that there was some kind of conspiracy. This was during the civil war when he was a prisoner of the Free State:

In July, 1922,I was a Republican prisoner in Mountjoy Jail and attempted to escape. Within sight of freedom, I was recaptured and for nearly three weeks I was held in solitary confinement in a basement punishment cell. Towards the end of my punishment period I was transferred at midnight in an armoured car to Kilmainham Jail, where I was again allowed to mix with other military prisoners. Here I made another unsuccessful attempt to escape, but because of the humanity of the Governor, Sean O'Mhuirthille, there was no further punishment. I was talking with some other prisoners on the night of August 22nd, 1922, when the news came in that Michael Collins had been shot dead in West Cork. There was a heavy silence throughout the jail, and ten minutes later from the corridor outside the top tier of cells I looked down on the extraordinary spectacle of about a thousand kneeling Republican prisoners spontaneously reciting the Rosary aloud for the repose of the soul of the dead Michael Collins, President of the Free State Executive Council and Commander-in-Chief of the Free State Forces. There was, of course, little logic in such an action, but I have yet to learn of a better tribute to the part played by any man in the struggle with the English for Irish Independence. Through all the hates and bitterness of Civil War, those Republican prisoners, remembered that the dead leader, latterly their enemy, was once an inspiration and driving force in their struggle with the alien army of occupation.
Here I may as well also kill the canard that the I.R.A. plotted "and planned Collins' death in 1922 and in fact assassinated him. About a week after his death we were transferred from Kilmainham to Gormanstown and on the very first day there I succeeded in escaping. Ten or eleven days later I walked into West Cork and interviewed the men who had fired the shots, one of which had ended the life of Michael Collins. The facts of his death were that a West Cork Column was lying in ambush for several days on the Bandon-Macroom road to attack the Free State troops who periodically used that route. On August 22nd, 1922, this Column waited for several hours at Bealnablath, but in the afternoon, having decided it was unlikely that the target would pass that day, the order to withdraw was given. The main body of the Column had retired over a mile and the small rearguard over a quarter of a mile from the ambush position, when a Free State convoy appeared. The main Column was out of sight and range, but the small rear- guard turned and opened fire from nearly five hundred yards range at the passing convoy which immediately stopped. The Free State party dismounted and lying on the road returned the fire, but the rearguard after firing less than a dozen rounds hurried on after the main body. One of those long range shots had killed Michael Collins, the only one of his party to be hit. It was almost five hours later when the I.R.A. Column first heard that Collins had been with the Free State convoy and that he had been killed in the skirmish with the Column's rearguard.

Rog