Not an exhaustive list of books by any measure but enough to get anybody started and familiar. You can acquire the vast majority: https://www.connollybooks.org
Labour, Nationality and Religion by James Connolly
The Re-conquest of Ireland by James Connolly
Labour Nationality and Religion by James Connolly
Socialism Made Easy by James Connolly
Life of Wolfe Tone by Thomas Bartlett
James Connolly lays the foundations of Socialist Republican theory for Ireland with an anti-imperialist and anti-capitalist grasp of the material conditions of Ireland in the early 20th century. James Connolly perfectly outlines the role of the Church, the national bourgeois (who only want to become the new ruling class of Ireland rather than participate in the liberation of the Irish people). James Connolly and his analysis forms the crux of virtually every political organisation subscribed to Socialist Republican/Marxist politics in Ireland.
Fianna Fail and Irish Labour by Kieran Allenn
The IRA 1926-1936 by Brian Hanley
The Lost Revolution by Brian Hanley and Scott Millar
Liam Mellows and the Irish Revolution by C Desmond Greaves
Tom Barry by Meda Ryan
The Provisional IRA: From insurrection to parliament
Sean Murray: Marxist-Leninist and Irish Socialist Republican
Theobald Wolfe Tone and the Irish Nation
This should really give the reader a general overview of Socialist Republican ideas and subsequently inform the reader on the various events post-civil war and developments within the IRA and Republican movement.