Davis is the son of a Welsh father, a surgeon in the Royal Artillery, and an Irish mother. Through his mother he is descended from the Gaelic noble family of O’Sullivan Beare. His father dies one month after his birth and his mother moves to Warrington Place near Mount Street bridge in Dublin. In 1830, they move to 67 Lower Baggot Street. He attends school in Lower Mount Street before studying at Trinity College, Dublin. He graduates in Law and received an Arts degree in 1836, before being called to the Irish Bar in 1838.
Davis gives a voice to the 19th-century foundational culture of modern Irish nationalism. Formerly it is based on the republicans of the 1790s and on the Catholic emancipation movement of Daniel O’Connell in the 1820s-30s, which has little in common with each other except for independence from Britain. Davis aims to create a common and more inclusive base for the future. He establishes The Nation newspaper with Charles Gavan Duffy and John Blake Dillon.
He writes some stirring nationalistic ballads, originally contributed to The Nation and afterwards republished as Spirit of the Nation, as well as a memoir of Curran, the Irish lawyer and orator, prefixed to an edition of his speeches, and a history of King James II‘s parliament of 1689. He has formed many literary plans which are unfinished at the time of his early death.
Davis supports O’Connell’s Repeal Association from 1840, hoping to recreate the old Parliament of Ireland. They split during a debate on the proposed new Queen’s University of Ireland, when Davis is reduced to tears by O’Connell’s superior debating skill. Davis is in favour of a university that would inclusively educate all Irish students. O’Connell and the Catholic hierarchy prefer a separate system for Catholic students within Ireland that would remain under church control.
O’Connell generally refers to his inexperienced allies as “Young Ireland,” initially as a dismissive term, that from the 1870s becomes the accepted term for nationalists inspired by Davis. He also prefers a federal arrangement with Britain in the 1840s while Davis seeks a greater degree of autonomy. Both agree that a gradual and non-violent process is the best way forward. Despite their differences, O’Connell is distraught at Davis’s early death.
Davis is a Protestant, but preaches unity between Catholics and Protestants. To him, it is not blood that makes a person Irish, but the willingness to be part of the Irish nation. He is to the fore of Irish nationalist thinking and it has been noted by later nationalist notables, such as Patrick Pearse, that while Theobald Wolfe Tone laid out the basic premise that Ireland as a nation must be free, Davis is the one who built this idea up promoting the Irish identity.