1851 in Ireland

Events

• 30 March – the United Kingdom Census shows that, as part of the legacy of the Great Famine, the population of Ireland has fallen to 6,575,000 - a drop of 1,600,000 in ten years. This is the first census to note use of the Irish language.
• 1 August – Midland Great Western Railway extended from Mullingar to Galway.
• 7 August – Poor Relief Act provides for the establishment of dispensaries.
• Construction of MacNeill's Egyptian Arch, a railway bridge near Newry on the Dublin-Belfast railway line is completed.
• Tillie and Henderson open their first shirt factory in Derry.

Undated

• Ancestors of Presidents of the United States Barack Obama and Joe Biden migrate from Ireland to the U.S.
• Jacob's brand of biscuits, bars and coffee is founded; their first biscuit is possibly Goldgrain.

Births

• 8 January – William McDonnell, 6th Earl of Antrim, peer.
• 14 March – Paddy Ryan, boxer.
• 17 March – Ted Sullivan, Major League Baseball player and manager.
• 21 March – Henry Prittie, 4th Baron Dunalley, peer and Lord Lieutenant of County Tipperary 1905–1922.
• 4 April – James Campbell, 1st Baron Glenavy, lawyer, Lord Chancellor of Ireland, first Chairman of Seanad Éireann.
• 22 May – William Moxley, representative from Illinois's 6th congressional district.
• 4 September – John Dillon, land reform agitator, Irish Home Rule activist, nationalist politician, MP and last leader of the Irish Parliamentary Party.
• 3 December – George Noble Plunkett, nationalist, politician, museum curator.

Deaths

• 23 May – Richard Lalor Sheil, politician, writer and orator.
• 1 September – Anne Devlin, Irish republican and housekeeper to Robert Emmet.

Census