The National Flag of Ireland (Irish: An Bhratach Náisiúnta), also known as the Irish tricolour, was adopted officially in 1919 by the the state called Ireland, sometimes known as the Republic of Ireland. The tricolour was given constitutional status under the 1937 Constitution of Ireland. In the English language, when referring to the Irish tricolour it is correctly pronounced as try-colour (with emphasis on the first syllable) as opposed to trickalour for the French and other flags.

The tricolour is regarded by many nationalists as the national flag of the whole of island of Ireland. Thus it is flown (often controversially) by many nationalists in Northern Ireland as well as by the Gaelic Athletic Association. Historically Ireland has been represented by a number of other flags, including Saint Patrick's cross, and the "four provinces" flag.

The tricolour, with its 3 equal vertical bands (pales) of green (hoist side), white, and orange was first flown from the Wolfe Tone Club, on The Mall in Waterford City, on 7 March 1848 by Thomas Francis Meagher. Apart from the constitutional status of the flag there are no additional statutory provisions on the flag's construction but the colours of the flag, under the Pantone Matching System, are green (Pantone 347), white and orange (Pantone 151). It was first used by Irish nationalists in 1848 during the Young Irelanders' rebellion, though the colours on the original flag were in reverse order to the modern version. Inspired by the French tricolour and the Newfoundland Tricolour (Meagher's father was born in Newfoundland), it was designed to represent the Nationalist (mainly Roman Catholic) majority (represented by green) and the Unionist (mainly Protestant) minority (represented by orange due to William of Orange) living together in peace (symbolised by the white band).

In numerous songs and poems, the colours are enumerated as "green, white and gold." Flags based on this will have a yellow stripe rather than an orange one. This is actively discouraged by the state. Having "gold" in place of "orange" may variously be interpreted as simple poetic license, a throwback to the green and gold flag of nineteenth century nationalism, or a desire to downplay the symbolism of "green" Ireland being in harmony with Orangeism.

Contrary to myth, the tricolour was not the actual flag of the Easter Rising, although it had been flown from the GPO; that flag was in fact a green flag with a harp and the words "Irish Republic". However the tricolour became the flag of the Irish Republic declared in 1919 after the election of the First Dail.

The 1922 Free State constitution did not provide for national symbols. The modern Constitution of Ireland provides in Article 7 that the "national flag is the tricolour of green, white, and orange". Today, the European flag is flown alongside the national flag on all official buildings, and in most places where the Irish flag is flown over buildings. The National Flag is flown over the following buildings:

• Áras an Uachtaráin (the residence of the President of Ireland, see Presidential Standard)
• Leinster House (the seat of the Irish parliament), when parliament is in session.
• Irish courts and state buildings.
• Irish military installations, at home and abroad and Garda (police) stations.

The tricolour is also draped across the coffins of:

• Presidents and ex-presidents of Ireland.
• Soldiers and Garda (police) personnel killed in the line of duty
• Other notables accorded state funerals, such as Roger Casement in 1965, or Kevin Barry in 2001.

Use in Northern Ireland

The purported symbolism of the flag (peace between Catholics and Protestants) has not become a universal reality. In 1920, Ireland was partitioned, with the unionist-dominated northeast becoming Northern Ireland, while later, in 1922, the remainder of Ireland left the United Kingdom of Great Britain and Ireland to form the Irish Free State.

Northern Ireland continued to use the British Union Flag and created its own derivation of the flag of Ulster (with a crown on top of a six pointed star) to symbolise the state. Furthermore, for many years the tricolour was effectively banned in Northern Ireland under the Flags and Emblems (Display) Act (Northern Ireland) 1954 which empowered the police to remove any flag that could cause a breach of the peace but specified rather controversially that a Union flag could never have such an effect. In 1964, the enforcement of this law by the RUC at the behest of Ian Paisley, involving the removal of a single tricolour from the offices of Sinn Féin in Belfast, led to two days of rioting. The tricolour was immediately replaced, highlighting the difficulty of enforcing the law.

Despite its original symbolism, in Northern Ireland the tricolour, along with most other markers of either British or Irish identity, has come to be a symbol of division. The Ulster Unionist Party Government of Northern Ireland adopted the Flag of Northern Ireland (based on the flag of Ulster) in 1953. Thus it is this flag and the Union Jack that are flown by unionists, while the tricolour is often used to represent nationalist defiance.

In Northern Ireland, each community uses its own flags, murals and other symbols to declare its allegiance and mark its territory, often in a manner that is deliberately provocative. Kerb-stones in unionist and loyalist areas are often painted red, white and blue, while in nationalist and republican areas kerb-stones may be painted green, white and orange. Elements of both communities fly "their" flag from chimneys and tall buildings.

Under the 1998 Belfast Agreement, flags continue to be a source of disagreement in Northern Ireland. The Agreement states that:
All participants acknowledge the sensitivity of the use of symbols and emblems for public purposes, and the need in particular in creating the new institutions to ensure that such symbols and emblems are used in a manner which promotes mutual respect rather than division.

Nationalists have pointed to this to argue that the use of the Union Flag for official purposes should be restricted, or that the tricolour should be flown alongside the British flag on government buildings. Unionists argue that the recognition of the "principle of consent" (i.e. that Northern Ireland's constitutional status cannot change without a majority favouring it) by the signatories amounts to recognising that the Union Flag is the only legitimate official flag in Northern Ireland.

Nonetheless, some level of compromise has been achieved. The British flag is no longer flown over Parliament Buildings and state offices except on a limited number of 'named days' (honouring, for example Queen Elizabeth II's official birthday). Major exceptions to this rule are the City Hall in Belfast (where the Union Flag is allowed to fly year-round), and the local district councils, for instance Lisburn council, which can choose to fly the Union Flag every day of the year or not at all (for instance Down District Council). A Sinn Féin Lord Mayor of Belfast (Alex Maskey) displayed both flags in his own offices, and this caused much controversy.