Michael Collins was born at Woodfield, Clonakilty, in County Cork in 1890. He was the third son and the youngest of eight children. His father, also called Michael Collins, was 75 years of age when Michael junior was born. On his deathbed the father pointed to his youngest child and urged his grieving family to mind Michael because, 'One day he'll be a great man. He'll do great work for Ireland.' Michael was 6 years of age at the time.

During those six years Michael had been greatly influenced by his father, who encouraged his children to learn patriotic ballads and poetry. West Cork was the heartland of Fenianism, the Irish nationalist movement founded in the 19th century. Jermiah O'Donovan Rossa, one of its founders, had been a teacher in a school in Rosscarberry, three miles away from the Collins household. Michael's own teacher, Denis Lyons, was a member of the Fenian organisation, the IRB (Irish Republican Brotherhood) and was to prove an inspirational figure. The local blacksmith, James Santry, was also a Fenian. Young Michael would often call to his forge to hear stories of earlier Irish rebellions in 1798 and 1848. Years later Michael Collins was to recall that, "In Denis Lyons and James Santry I had my first tutors capable of, because of their personalities alone, infusing into me pride of the Irish as a race." As a child Michael also read widely. He was familiar with Shakespeare and the great novelists of the 19th century. Every week he read the nationalist newspapers "The Freeman's Weekly" and "The Leader". When only 11 years of age Michael began to subscribe to 'The United Irishman', edited by Arthur Griffith. Almost 20 years later, Griffith and Michael Collins would be the most important Irish representatives in the Treaty negotiations with Britain. Griffith was the founder of Sinn Fein, a nationalist party that exists to this day. At that time Sinn Fein was not a republican party. Griffith believed that a Republic was unattainable and that Home Rule, which the constitutional nationalists sought, was inadequate. His goal was an independent Ireland with the same monarch as England. While this did not accord with the Fenian view, Griffith did have a profound influence on the young Michael Collins. At the age of 12 he wrote, "In Arthur Griffith there is a mighty force in Ireland. He has none of the wildness of some I could name. Instead there is an abundance of wisdom and an awareness of things which are Ireland."

The young Michael Collins was a keen sportsman. He played the local game of road bowling. He enjoyed the Irish game hurling, and was fond of fishing. But it was his prowess as a wrestler for which Michael was noted. He took on all comers, and rougher bouts usually ended with Collins biting the ear of his opponent. Later during the War of Independence he would often break the mounting tension by insisting on "a bit of ear", as he called it, with his comrades.

In July 1906, at the age of 15, Michael Collins emigrated to London where he worked as a boy clerk in West Kensington Post Office. He quickly found his feet in the strong Irish community in London. He joined the Gaelic Athletic Association(GAA), the Gaelic League, which promoted the revival of the Irish language, and Sinn Fein. In November 1909 he was inducted into the Irish Republican Brotherhood (IRB). At this time the IRB was in decline, but the failure of the Irish Parliamentary Party (IPP) to achieve Home Rule through constitutional means attracted younger members to the organisation. Shortly after joining the IRB Michael left the Post Office and took up a post with a stockbroking company and later he worked in the Whitehall Labour Exchange. Finally, before returning to Ireland, he worked briefly with an American firm, the Guaranty Trust Company.

Outside of work Collins wrote papers on Irish history and current political events. The Irish question had now moved to the centre of the political stage. The election results of 1910 gave the IPP the balance of power and its leader, John Redmond, demanded the introduction of Home Rule. The Home Rule Bill, which came before the House of Commons in April 1912, met with stiff resistance from the Ulster Unionists. Under the Home Rule scenario they feared that the Protestant culture would lose out to the Catholic nationalist majority. In 1913 the Ulster Unionist leader, Sir Edward Carson, organised the Ulster militias into the Ulster Volunteer Force (UVF) and threatened to set up a provisional government in Belfast if Home Rule was introduced. Nationalists in Dublin responded by forming the Irish Volunteers. Although founded by Eoin O'Neill, a professor in University College Dublin, IRB members were active behind the scenes. Seeing the threat, Redmond managed to gain control of the executive of the Irish Volunteers. With the outbreak of the First World War, Redmond proposed in the House of Commons that the Volunteers and the UVF come together to defend Ireland against invasion. With the question of Home Rule now deferred until after the war, the Irish Volunteers split into two camps. The majority followed Redmond's advice and joined the British war effort in the hope of gaining Home Rule, while a minority dominated by the IRB stayed at home to organise armed rebellion.