Ferguson was born on 10 March 1810 at 23 High Street Belfast. Educated at Belfast Academical Institution and Trinity College, Dublin, he studied at Lincoln's Inn London, before being called to the Irish Bar in 1838. In 1848, he married Mary Guinness, a member ofthe brewing family, and their home at 20 North Great George's Street Dublin, became a hospitable focus of intellectual and artistic life. Ferguson practised on the North-East Circuit, taking silk in 1859.

As a young man, Ferguson contributed to Blackwood's Magazine, and a notable early poem was 'The Forging of the Anchor'. The influential Dublin University Magazine published his eerie poem 'The Fairy Thorn'. He met the poet James Clarence Mangan and scholars such as John O'Donovan and George Petrie, and drew extensively on Irish mythology for poems such as 'The Tain Quest' and 'The Death of Dermid'. Lays of the Western Gael (1865) and Congal (1872) opened up territory later explored by W. B. Yeats, who called Ferguson 'the greatest poet Ireland has produced'. More successful though, are love songs such as 'The Coolun' and 'The Lark in the Clear Air'.

Deeply interested in antiquities, Ferguson wrote many papers for the Royal Irish Academy, becoming its president in l882 .His major work, Ogham Inscriptions in Ireland, Wales and Scotland, was edited for posthumous publication (1887) by his wife. Politically, he began as a unionist, but later founded the Protestant Repeal Association and sought to restore an Irish parliament. Although not a contributor to The Nation, his 'Lament for Thomas Davis' is a sympathetic tribute to its founder. However, his unease at nationalist violence was evident in 'At the Polo-Ground', a poem on the 1882 murder of the chief secretary and under-secretary in Phoenix Park, Dublin.

Ferguson gave up his legal practice in 1867, becoming deputy keeper of the public records of Ireland. His thorough reorganisation of a neglected department was recognised in a knighthood in 1878. He died at Howth, Co Dublin, on 9 August 1886.

See Plaque at Ferguson's Dublin home.