1840 in Ireland

Events

• 10 January – Uniform Penny Post introduced.
• 1 April – Theatre Royal, Cork burns down.
• 19 May – foundation stone of the Roman Catholic St Mel's cathedral, Longford, is laid.
• 10 July – General Assembly of the Presbyterian Church in Ireland established.
• 28 July – first permanent presence of The Church of Jesus Christ of Latter-day Saints in Ireland when Mormon missionaries John Taylor, William Black and James McGuffie arrive to work in the Newry, Lisburn and Belfast areas. On 31 July Thomas Tait becomes the first convert baptised in Ireland, at Loughbrickland.
• The Palm House in Belfast Botanic Gardens is completed, constructed by Richard Turner of Dublin. It is one of the earliest examples of a curvilinear cast iron glasshouse in the world.
• Bewley's established as tea and coffee importers.

  Arts and literature

• Edward Bunting's The Ancient Music of Ireland is published, incorporating "A Dissertation on the Irish Harp and Harpers, Including an Account of the Old Melodies of Ireland".

  Births

• 1 January – Patrick Walsh, journalist, politician and mayor of Augusta, Georgia.
• 4 January – Bishop Richard Owens, Bishop of Clogher 1894–1909.
• 8 January – Henry Arthur Blake, British colonial administrator and Governor of Hong Kong.
• 17 January – William Pery, 3rd Earl of Limerick, peer.
• 27 February – Thomas Kelly-Kenny, British Army general who served in the Second Boer War.
• 29 February – John Philip Holland, engineer, developed the first Royal Navy submarine.
• 25 March – Myles Keogh, officer in American Civil War, later in U.S. 7th Cavalry Regiment, killed at the Battle of the Little Bighorn.
• 26 April – Paddy Hannan, gold prospector whose discovery in 1893 near Kalgoorlie, Western Australia set off a gold rush.
• 27 April – Tom Gallaher, tobacco manufacturer.
• 23 May – George Throssell, second Premier of Western Australia.
• 20 September – Ellen Mary Clerke, author, journalist, poet and science writer.
• 1 November – Arthur Guinness, 1st Baron Ardilaun, businessman, politician, and philanthropist.
• 17 November – Lawrence Parsons, 4th Earl of Rosse, eighteenth Chancellor of Trinity College, Dublin.
• 28 December – Thomas Hovenden, artist and teacher.
• ;Full date unknown
• :*William Abraham, Irish Nationalist MP.
• :*Timothy J. Campbell, Democrat U.S. Representative from New York.
• :*Timothy H. O'Sullivan, photographer in the United States

  Deaths

• 15 April – Thomas Drummond, military surveyor and Under-Secretary for Ireland.
• 21 April – Standish O'Grady, 1st Viscount Guillamore, Lord Chief Baron of the Exchequer in Ireland.
• 12 June – Gerald Griffin, novelist, poet and playwright.
• 20 August – George Canning, 1st Baron Garvagh, politician.