Connolly became a socialist and in 1896 moved to Dublin as an organizer of the Dublin Socialist Society. Later he founded the Irish Socialist Republican Party and established The Workers' Republic in 1898. Books by Connolly during this period included Erin's Hope (1897) and The New Evangel (1901).
In 1902 Connolly returned to Edinburgh and helped launch the journal, Socialist, in Edinburgh. Connolly was influenced by the writings of the American radical Daniel DeLeon and published several articles by him in his journal.
Connolly emigrated to the United States in 1903. He established the Irish Socialist Federation and the newspaper, The Harp. In 1905 Connolly joined the Industrial Workers of the World (IWW) and worked hard at building up the movement in Newark, New Jersey. He also published several books including Socialism Made Easy (1909), Labour in Irish History (1910) and Labour, Nationality and Religion (1910).
In 1910 Connolly returned to Dublin where he joined the Socialist Party of Ireland. The following year William O'Brien arranged for Connolly to become organizer of the Irish Transport and General Workers Union in Belfast. In 1912 Connolly and James Larkin established the Irish Labour Party.
By 1913 the Irish Transport and General Workers' Union had 10,000 members and had secured wage increases for most of its members. Attempts to prevent workers from joining the ITGWU in 1913 led to a lock-out. Connolly returned to Dublin to help the union in its struggle with the employers. This included the formation of the Irish Citizen Army. Despite Larkin raising funds in England and the United States, the union eventually ran out of money and the men were forced to return to work on their employer's terms.
Connolly took control of the Irish Transport and General Workers' Union when James Larkin left for a lecture tour of the United States in October 1914. He also revived his socialist journal, The Workers' Republic, but it was suppressed in February 1915. Later that year he published The Re-Conquest of Ireland (1915).
During the Easter Rising, Connolly's Irish Citizen Army fought alongside the Irish Volunteers under Patrick Pease. Connolly served in the General Post Office during the fighting and was severely wounded. James Connolly was executed on the 12th of May 1916