Reilly is the son of a solicitor and completes his education at Trinity College, Dublin. From early on he espouses the republican beliefs of Theobald Wolfe Tone and Robert Emmet and writes for The Nation and John Martin‘s The Irish Felon in support of economic and political improvements for the working class. He is more interested in the realities of the common man than high idealism.
As a member of the Irish Confederation during the Great Famine, Reilly together with John Mitchel and James Fintan Lalor advocate the refusal to pay rents, retention of crops by small tenant farmers and labourers to feed their own families, and the breaking up of bridges and tearing up of railway lines to prevent the removal of food from the country.
Reilly is involved in the failed Young Irelander Rebellion of 1848 and is forced to flee to the United States where he becomes active in U.S. political affairs in support of Irish independence. He is reported to be the founder of The People newspaper in New York City which folds after six months in 1849.
James Connolly claims that as the editor of the Protective Union labour rights newspaper for the printers of Boston, Reilly is a pioneer of American labour journalism and that Horace Greeley believed of his series of articles in The American Review on the European situation “that if collected and published as a book, they would create a revolution in Europe.”
It is possible that Connolly confuses The United States Magazine and Democratic Review, which is known for its political activism, with The American Review, which for a time had Edgar Allan Poe as an editorial assistant. Other sources refer to Reilly as being editor of the New York Democratic Review and later the Washington Union.