Events from the year 1866 in Ireland.

• 28 January – the Midland Great Western Railway opens to Westport railway station.
• 22 June – Archbishop Cullen is elevated to the cardinalate as the first Irish Cardinal.^[1]
• 13 July – the SS Great Eastern sets out from Valentia Island on the second (successful) attempt to lay the transatlantic telegraph cable.^[2] Robert Halpin is master and William Thomson technical consultant.
• 14 October – St Peter's Church, Belfast, later to become the Roman Catholic Cathedral, is dedicated, although the building is incomplete.
• Maziere Brady retires as Lord Chancellor of Ireland, an office to which he was appointed in 1846.^[3]
• Alexandra College is founded at Milltown, Dublin by the Quaker Ann Jellicoe, the first women's college in Ireland to aim at a university-level education.

• The first modern Irish Derby, created by the 3rd Earl of Howth, the 3rd Marquess of Drogheda and the 3rd Earl of Charlemont, is run at the Curragh Racecourse.
• The Ulster Yacht Club is revived at Bangor, County Down, on the initiative of Frederick Temple Blackwood, 5th Baron Dufferin and Claneboye.^[4]

• The ballad "Come Back to Erin" is composed by 'Claribel', the English songwriter Charlotte Alington Barnard.

• 5 February – Domhnall Ua Buachalla, member of 1st Dáil, Fianna Fáil TD, last Governor-General of the Irish Free State (died 1963).
• February – Michael Egan, trade unionist, city councillor and Cumann na nGaedheal TD (died 1947).
• 15 June – Charles Wood, composer (died 1926).
• 13 July – Emily Winifred Dickson, gynaecologist (died 1944 in Liverpool).
• 16 August – Dora Sigerson, poet (died 1918 in London).
• 1 November – Cheiro, born William John Warner, astrologer (died 1936 in the United States).
• 3 December – Ethna Carbery, born Anna Johnston, writer and poet (died 1902).
• 7 December – Maude Delap, marine biologist (died 1953)
• December – Thomas Byrne, recipient of the Victoria Cross for gallantry in 1898 at the Battle of Omdurman, Sudan (died 1944).

  Full date unknown

  • Éamon a Búrc, tailor and seanchaí (died 1942).
  • Master McGrath, greyhound (died 1873).
  • Bridget Sullivan, domestic housemaid for Borden family of Fall River, Massachusetts (died 1948 in the United States).

• 5 January – Augustus Warren Baldwin, naval officer and political figure in Upper Canada (born 1776).
• 11 January – Gustavus Vaughan Brooke, actor (born 1818).
• 17 January – George Petrie, painter, musician, antiquary and archaeologist (born 1790).
• 4 March – Alexander Campbell, religious leader in Britain and the United States (born 1788).
• 18 May – Francis Sylvester Mahony, humorist and poet (aka Father Prout) (born 1804).
• 26 October – John Kinder Labatt, brewer in Canada (born 1803).
• 26 October – Patrick McHale, soldier, recipient of the Victoria Cross for gallantry in 1857 at Lucknow, India (born 1826).

  Full date unknown

  • Edward Eagar, lawyer and criminal transported to Australia, politician (born 1787; died in London).

• 1866 in Scotland
• 1866 in Wales