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 (Ireland) 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.
8 January - William McDonnell, 6th Earl of Antrim, peer (died 1918).
14 March - Paddy Ryan, boxer (died 1900).
21 March - Henry Prittie, 4th Baron Dunalley, peer and Lord Lieutenant of County Tipperary 1905-1922 (died 1927).
4 April - James Campbell, 1st Baron Glenavy, lawyer, Lord Chancellor of Ireland, first Chairman of Seanad Éireann (died 1931).
22 May - William Moxley, representative from Illinois' 6th congressional district (died 1938).
4 September - John Dillon, land reform agitator, Irish Home Rule activist, nationalist politician, MP and last leader of the Irish Parliamentary Party (died 1927).
George Noble Plunkett, nationalist, politician, museum curator (died 1948).
Ted Sullivan, Major League Baseball player and manager (died 1929).
23 May - Richard Lalor Sheil, politician, writer and orator (born 1791).
1 September - Anne Devlin, Irish republican and housekeeper to Robert Emmet (born 1780).
1851 in Ireland Wikipedia
Events from the year 1851 in Ireland.