G Division (Dublin Metropolitan Police)
By 1859 much of the G division's work was concerned with Fenianism. Superintendent Daniel Ryan headed the detectives answering to Sir Henry Lake, chief commissioner of the Dublin Metropolitan Police (DMP). Ryan had an informer named Pierce Nagle within the offices of the Fenian Irish People newspaper. In 1865 Nagle warned Ryan about an “action this year” message on its way to the Irish Republican Brotherhood unit in Tipperary. In 15 July 1865 American-made plans for an IRB rising in Ireland were discovered when the emissary lost them at Kingstown railway station. Ryan raided the offices of the newspaper on 15 September and the staff were arrested. They were tried and sentenced to terms of penal servitude.
In 1874 John Mallon succeeded Ryan as head of the Dublin detective service. Mallon's father had been linked with the Ribbon Society but the son had specialised in his career working against Fenianism. He had an extensive knowledge of the subversives and operated a personal network of spies and informers. In the 1880s G Division was pitted against separatist insurgents including the Invincibles, the Land League and even the Irish Parliamentary Party, arresting Charles Stewart Parnell in 1881. Mallon supervised G division until his retirement in January 1902. To protect his informants, Mallon had refused to commit much of his knowledge to paper.
Many DMP officers actively assisted the IRA during the Irish War of Independence (1919–21), most famously Edward Broy, who passed valuable intelligence to Michael Collins throughout the conflict. Broy was a double agent with the rank of Detective Sergeant (DS)  and worked as a clerk inside the G division branch. While there he copied sensitive files for IRA leader Michael Collins and passed many of these files on to Collins through Thomas Gay, the librarian at Capel Street Library. On 7 April 1919, Broy smuggled Collins into G Division's archives in Brunswick Street, enabling him to identify "G-Men", six of whom would be killed by the IRA.
- July 30, 1919, the first assassination authorised by Collins was carried out when Detective Sergeant Patrick Smith, "the Dog", was shot near Drumcondra, Dublin. Collin's Squad would continue targeting plainclothes G-men.
- September 12, 1919 Detective Daniel Hoey of DMP "G" Division killed by Michael Collins "The Squad"
- October 19, 1919 Detective Michael Downing of DMP "G" Division killed
- November 29, 1919 Detective Sgt John Barton of DMP "G" Division killed by Michael Collins "The Squad"
- January 21, 1920 RIC District Inspector William Redmond of DMP "G" Division killed by Michael Collins "The Squad"
- April 14, 1920 saw the shooting of Detective Constable Harry Kells in Camden St Portobello, Dublin. He was rushed to the Meath Hospital where he died. Kells had been carrying out identity parades among the many republican inmates in Mountjoy Prison. Over 100 people were arrested as a result.
- April 20, 1920 Detective Laurence Dalton of "G" Division shot and killed
Another DMP "G" Division spy for Collins was David Neligan. In November 1923 the division was merged with "Oriel House", the Free State Intelligence Department. The new Detective Branch was put under the control of Colonel Neligan now Director of Intelligence in the Free State Army.
- Patrick Maume, ‘Mallon, John (1839–1915)’, Oxford Dictionary of National Biography, Oxford University Press, 2004
- Christy Campbell, p.58-9
- Cottrell, Peter (2006). The Anglo-Irish War: The Troubles of 1913-1922. Osprey Publishing. p. 66. ISBN 1846030234.
- The Irish War: The Hidden Conflict between the IRA and British Intelligence by Tony Geraghty (ISBN 978-0801871177), page 336
- Cottrell, op. cit., pg 53.
- Mackay, James. Michael Collins: A Life
- Kavanagh Family
- New York Times, 16 April 1920