From "Remembering Limerick by Des Ryan
Richard Bennett, in 1961, he stated in an article in The New Statesman that he had uncovered the identity of the assassin; his name, he said, was George Nathan, who had died in 1937 during the Spanish Civil War fighting on the side of the Republicans in the XV International Brigade.
Bennett got his information about George Nathan from an unnamed source, two ex-British army officers, whom we shall call, for the purpose of this article, officer 'A' and officer 'B'. Both men were former members of the Auxiliary Division of the R.I.C.
Officer 'A' told Bennett that Nathan had come down from Dublin to the Auxiliary depot at Killaloe, and said he had a job to do that night. Nathan asked officer 'A' to drive into Limerick with him, and have dinner with him at Kidd's restaurant in O'Connell Street. Officer 'A' said he did not go but that another Auxiliary, officer 'B', went with Nathan. Nathan and his companion arrived back to the depot at six o'clock the next morning, drunk and pale in the face. At breakfast Nathan told the Auxiliaries that he had killed Clancy and O'Callaghan. Officer 'A' had lost touch with Nathan over the years, but he knew the name of Nathan's accomplice.
After some detective work Bennett tracked down officer 'B', an Irishman living in a small flat in Bayswater, London. Officer 'B' said at first that he did not know Nathan. When asked if a Colonel Andrews was his commanding officer at Killaloe, it seemed to unlock the secrets from his past, and he said yes, he did remember Nathan. Officer 'B's' story was that Nathan had picked him up at Kidd's Restaurant to go on a raid, but instead they had knocked off a couple of Shinners.
In 1932, Brigadier-General F. P. Crozier, a former commandant of the Auxiliaries, in his book Ireland Forever, stated that after he carried out a personal investigation of the murders in March 1921 he had come to the same conclusion as Mrs. O'Callaghan, that is, that her husband 'was murdered by police, acting under orders, as part of a plan to 'do away with' Sinn Fein leaders, and put the blame on Sinn Fein
A further twist to the story of the Murdered Mayors was added when, in February 1982, the Limerick Leader published a picture of 'Black and Tans', taken at William Street Police Station (now demolished). A side note to the photograph by WiIlie 'Whack' Gleeson, alleged that two of the Tans in the photograph, Sergeant Leech and Sergeant Horan, were also involved in the killings. While Leech was implicated in the murder of Joseph O'Donoghue, we don't know what part, if any, Horan played on the night in question. In the summer of 1922 Leech was shot dead at Harcourt Street Station, Dublin.
Glesson claims that these killings were carried out by two Auxiliaries based in Dublin - one was George Nathan.
A number of British Officers and Auxiliaries who operated in the Limerick area were believed to have taken part in after dark murders. These officers included George Montagu Nathan an Auxiliary, formerly of the Warwickshire Regiment, Captain William Davis of the Machine Gun Corps (Davis was certainly a army not Auxiliary officer), Lieutenant Leslie Ibbotson also an Auxiliary and formerly of the Leicester Regiment. Nathan was implicated in the shooting of Tom Blake on the Clyde Road in early 1921. Both Nathan and Ibbotson were believed to have taken part in the murder of the two mayors of Limerick in March 1921.
Captain W Davis was identified, in a British Court of Inquiry, as the man who shot Richard Leonard at Boskill near Caherconlish on December 30th 1920. Charges against Davies were later dropped but his guilt was established. The two other army officers with him were Major William Grey and Lt G. M. Loup. It appears that they were drunk when Davis shot Leonard, and that it was not a "political" killing. I would discount Davis from the Lord Mayors' deaths
RIC men who were known or strongly suspected of having taken part in murders in the Limerick region were Sergeant William Leech, Sergeant James Horan and Sergeant Eugene Igoe. Leech was identified as having led the squad of men who murdered Joseph O'Donoghue on the same night that George Clancy and Michael O'Callaghan were murdered. William Leech was later shot dead at Westland Row railway station in Dublin during the Truce.
2012 Limerick Leader published their in depth report on the shootings.