A website called irishmedals.org mentions Martan o'Hogain's murder. It seems to have been one of a series of murders that took place between 1922 and 1923 after the killing of Michael Collins in August 1922.
On the 22nd of August 1922 the Criminal Investigation Department (C.I.D.) was formed, it was separate from any of the existing police forces and under the control of the Minister for Home Affairs. Members for the new force were drawn from the Army and Republican Police and were based in Oriel House, Westland Row, Dublin. The force consisted of over one hundred men and three women, the women were officially employed as secretaries but were in fact employed to deal with women engaged in hostilities against the State. The CID was allegedly responsible for the murder of anti-treaty personnel, and this included Martan o'Hogain.
I.R.A. Anti-Treaty Captain Martin Hogan was abducted by a group of about ten men from Eccles Place Dublin on the 21st of April 1923, his body was found dumped at Grace Park Road Dublin the next day."
Another website - http://www.theirishstory.com outlines that in November 1922, the Free State government passed emergency legislation allowing for the execution (with the signature of any two army officers) of anyone captured bearing arms against the state. In December 1922, they began official executions in Dublin, with the shooting of four low ranking IRA members, followed closely by Erskine Childers, who had been the republican head of propaganda.
It states that Martin Hogan, an Anti-Treaty IRA officer was arrested, killed and dumped, also in Whitehall, in April 1923. His girlfriend, who first tried to find him in one of the prisons, was told to enquire at Oriel House (CID headquarters), where she was told, “try the morgue”. His body was eventually found in a ditch in Drumcondra