20 April – Welsh pub landlords reported an increase in the number of Irish patrons visiting Wales where they could avoid the restrictions of Ireland's new smoking ban which prohibited smoking in Irish pubs. The drinkers could travel to Wales by ferryboat for as little as £10 for a day-return ticket, smoke cigarettes while drinking, and pay lower prices for their alcohol.[2][3]
May
1–25 May – Heads of government celebrated in Dublin as the European Union admitted ten new member states.
Local Elections, 2004 – Fianna Fáil's share of the vote fell sharply while all the other opposition parties made gains. Sinn Féin made a big breakthrough with a record number of councillors being elected.
7 August – Athlete Cathal Lombard was accused of taking performance-enhancing drugs at the Olympic Games.
13 August – Minister for Agriculture, Joe Walsh, announced his retirement from the Cabinet after seven years. He was the longest-serving agriculture minister in Europe.
16 November – Margaret Hassan was murdered by her captors in Iraq.
22 November – Bertie Ahern celebrated ten years as leader of the Fianna Fáil party.
December
15 December – The Irish Nationality and Citizenship Act 2004 was enacted. Inter alia, this provided that children born of most foreign national parents on the island of Ireland were no longer automatically entitled to Irish citizenship.[7][8][9]
16 December – In Colombia, the Penal Chamber of Bogotá's Supreme Tribunal handed down lengthy jail sentences to the Irish Colombia Three for training Colombian Marxist rebels.
18 December – The "Colombia Three", Niall Connolly, Martin McCauley and James Monaghan, jumped bail.
19 December – President McAleese convened a meeting of the Council of State to discuss the Health Amendment II Bill, which was presented the previous week by the Health Minister Mary Harney.
31 December – Taoiseach Bertie Ahern pledged €10 million in humanitarian aid to people affected by the earthquake and tsunami in South and Southeast Asia.
Shelbourne defeated KR Reykjavík in the first qualifying round on away goals. In the second qualifying round, Shelbourne lost the first leg 3–2 away to Hajduk Split, but two late goals in the home leg at Tolka Park meant they became the first Irish team to make it to the third qualifying round. After a 0–0 draw with Deportivo de La Coruña in front of 25,000 fans at Lansdowne Road, the Irish team lost 3–0 in Spain.
Bohemians and Longford Town suffered disappointing first qualifying round defeats to FC Levadia Tallinn and FC Vaduz respectively. Shelbourne entered the first round proper after their Champions League third qualifying round exit, but missed out on a place in the UEFA Cup group stages. After a 2–2 draw at Lansdowne Road, Shelbourne lost 0–2 in the return leg against French side Lille.
Cian O'Connor and the horse Waterford Crystal won gold for Ireland in the equestrian event. O'Connor was later stripped of this title because the horse tested positive for a prohibited substance.