O'Connell was born on 21 December 1887 in Ballina, County Mayo, to Jeremiah Ambrose and Winifred O'Connell. He was nicknamed "Ginger" because of his red hair. His father was a national school inspector, so the family lived in Sligo, Derry, Longford and Belfast, and Ginger attended a succession of primary schools. He studied in University College Dublin where he received a BA and a first class MA. He was a member of the Literary and Historical Society, and had an interest in boxing. He was living in Cavan with his father, his sister Mary Margaret, his brother John Aloysius and two servants, Mary Burke and Rose Anne O'Reilly, at the time of the 1911 census, when he was 23. He was working as a Solicitor's Apprentice, could read and write as well as speak both English and Irish, and was single. His mother was not living as it is recorded that his father was a widower.
O'Connell spent some time in the U.S. Army, serving with the 69th Infantry Regiment between 1912 and 1914. He returned to Ireland in 1914 and joined the Irish Volunteers, becoming Chief of Inspection in 1915. He travelled the country organising volunteer corps, as well as contributing to the Irish Volunteer's journal and delivering lectures on military tactics to both the Volunteers and Fianna Éireann. He also delivered a series of lectures about the famous Irish battles to the Gaelic League in Dublin. He was not a member of the Irish Republican Brotherhood as he believed that soldiers should not be a part of secret societies. At the time the 1916 Easter Rising, O'Connell was operating in Dublin under instruction from Joseph Plunkett. He was dispatched to Cork by Eoin MacNeill to try to prevent the Rising. Following the Rising, he was arrested and held in Frongoch internment camp from April to July 1916. In 1918 he was again arrested and interned, spending time in Wandsworth Prison with Arthur Griffith for the alleged involvement in the fabricated German Plot. During the Irish War of Independence, he was a member of the Irish Republican Army headquarters staff, as Assistant Director of Training and, after the killing of Dick McKee, as Director of Training. He coordinated, and was the principal lecturer, for a training course for military officers. The course was run clandestinely in the premises of the Topographical Society on Gardiner Street in Dublin. A sympathetic doorkeeper allowed O'Connell's group in at night when the society was not present. Topics delivered by O'Connell included tactics, ordinance and engineering.
In the IRA split after Dáil Éireann ratified the Anglo-Irish Treaty, O'Connell took the pro-Treaty side. He was made Deputy Chief of Staff in the National Army. On 26 June 1922, he was kidnapped by anti-treaty forces in reprisal for the arrest of an anti-treaty officer; his kidnapping was a precipitating factor in the formal outbreak of the Irish Civil War, when government pro-treaty forces two days later attacked anti-treaty forces occupying the Four Courts. O'Connell survived the fighting and spent the rest of the civil war as General Officer Commanding the Curragh Command. Following the Civil War, the National Army was reorganised, and as part of that O’Connell was demoted from general to colonel. He subsequently held a variety of positions: chief lecturer in the army school of instruction ; director of no. 2 bureau ; OC Equitation School ; quartermaster-general and director of the military archives. He also published articles on Irish and foreign military history and tactics in his time as a military historian. He married Gertrude McGilligan, and they had two children together - one son and one daughter. O'Connell died on 19 February 1944, of a heart attack.
