Dillon is born on May 5, 1814 in the town of Ballaghaderreen, on the border of counties Mayo and Roscommon. He is a son of Anne Blake and her husband Luke Dillon (d. 1826), who had been a land agent for his cousin Patrick Dillon, 11th Earl of Roscommon. His niece is Anne Deane, who helps to raise his family after his death.
Dillon is educated at St. Patrick’s College, Maynooth, leaving after only two years there, having decided that he is not meant for the priesthood. He later studies law at Trinity College, Dublin, and in London, before being called to the Irish Bar. It is during his time at Trinity College that he first meets and befriends Thomas Davis.
While working for The Morning Register newspaper Dillon meets Charles Gavan Duffy, with whom he and Davis found The Nation in 1842, which is dedicated to promoting Irish nationalism and all three men become important members of Daniel O’Connell‘s Repeal Association, which advocates the repeal of the Act of Union 1800 between Great Britain and Ireland.
The young wing of the party, of which they are key members with William Smith O’Brien and Thomas Francis Meagher, come to be known as Young Ireland and advocate the threat of force to achieve repeal of the Act of Union. This is in contrast to the committed pacifism of O’Connell’s “Old Ireland” wing. This posturing eventually leads to the Young Ireland rebellion of 1848 where a countryside devastated by the Great Famine fails to rise up and support the rebels.
According to fellow Irish nationalist, Justin McCarthy, “…it has been said of him that while he strongly discouraged the idea of armed rebellion, and had no faith in the possibility of Ireland’s succeeding by any movement of insurrection, yet when Smith O’Brien risked Ireland’s chances in the open field, he cast his lot with his leader and stood by his side in Tipperary.”
Dillon returns to Ireland on amnesty in 1855 and in 1865 is elected as a Member of Parliament for Tipperary. By now he advocates a Federal union of Britain and Ireland and denounces the violent methods advocated by the Irish Republican Brotherhood or Fenian movement.