Sinn Fin ( shin FAYN, Irish: [n fen]; English: "[We] Ourselves") is an Irish republican and democratic socialist political party active throughout both the Republic of Ireland and Northern Ireland.

The original Sinn Fin organisation was founded in 1905 by Arthur Griffith. Its members founded the First Dil and the revolutionary Irish Republic during the Irish War of Independence. The party split in the aftermath of the Irish Civil War, giving rise to the two traditionally dominant parties of Irish politics: Fianna Fil, and Cumann na nGaedheal (which became Fine Gael). The current Sinn Fin party took shape in 1970 after another split, the other faction eventually becoming the Workers' Party. During the Troubles, Sinn Fin was associated with the Provisional Irish Republican Army (IRA). For most of the conflict, there were broadcasting bans on Sinn Fin in the Irish media and in the British media. Although the party sat on local councils, it had a policy of abstentionism for the British parliament and the Irish parliament Dil ireann, standing for election but vowing not to take their seats. When Gerry Adams became party leader in 1983, electoral politics became increasingly important. In 1986, it dropped its abstentionist policy for the Dil; some members formed Republican Sinn Fin in protest. In the 1990s, Sinn Finunder the leadership of Adams and Martin McGuinnesswas involved in the Northern Ireland peace process. This led to the Good Friday Agreement and created the Northern Ireland Assembly. In 2006, it co-signed the St Andrews Agreement and accepted the Police Service of Northern Ireland.

Sinn Fin is the largest party in the Northern Ireland Assembly, having won the largest share of first-preference votes and the most seats in the May 2022 election, the first time an Irish nationalist/republican party has done so. From 2007 to 2022 it was the second-largest party in the Assembly, after the Democratic Unionist Party (DUP), and its members served as deputy First Minister in the power-sharing Northern Ireland Executive. In the UK House of Commons, Sinn Fin holds seven of Northern Ireland's seats; there, it continues to abstain from parliament. In Dil ireann it is the joint-largest party and is the main opposition, having won the largest share of first-preference votes in the February 2020 election.

The current party president is Mary Lou McDonald, who succeeded Gerry Adams in 2018.

Northern Ireland (Irish: Tuaisceart Éireann [ˈt̪ˠuəʃcəɾˠt̪ˠ ˈeːɾʲən̪ˠ] (listen); Ulster-Scots: Norlin Airlann) is a part of the United Kingdom that is variously described as a country, province, territory or region. Located in the northeast of the island of Ireland, Northern Ireland shares a border to the south and west with the Republic of Ireland. In 2011, its population was 1,810,863, constituting about 30% of the island's population and about 3% of the UK's population. The Northern Ireland Assembly (colloquially referred to as Stormont after its location), established by the Northern Ireland Act 1998, holds responsibility for a range of devolved policy matters, while other areas are reserved for the British government. Northern Ireland co-operates with the Republic of Ireland in several areas.Northern Ireland was created in 1921, when Ireland was partitioned by the Government of Ireland Act 1920, creating a devolved government for the six northeastern counties. The majority of Northern Ireland's population were unionists, who wanted to remain within the United Kingdom. They were generally the Protestant descendants of colonists from Great Britain. Meanwhile, the majority in Southern Ireland (which became the Irish Free State in 1922), and a significant minority in Northern Ireland, were Irish nationalists and Catholics who wanted a united independent Ireland. Today, the former generally see themselves as British and the latter generally see themselves as Irish, while a Northern Irish or Ulster identity is claimed by a large minority from all backgrounds.The creation of Northern Ireland was accompanied by violence both in defence of and against partition. During 1920–22, the capital Belfast saw major communal violence, mainly between Protestant unionist and Catholic nationalist civilians. More than 500 were killed and more than 10,000 became refugees, mostly Catholics. In the following decades, Northern Ireland had an unbroken series of Unionist Party governments. There was informal mutual segregation by both communities, and the Unionist governments were accused of discrimination against the Irish nationalist and Catholic minority, in what First Minister of Northern Ireland, David Trimble, called a "cold house" for Catholics. In the late 1960s, a campaign to end discrimination against Catholics and nationalists was opposed by loyalists, who saw it as a republican front. This unrest sparked the Troubles, a thirty-year conflict involving republican and loyalist paramilitaries and state forces, which claimed over 3,500 lives and injured 50,000 others. The 1998 Good Friday Agreement was a major step in the peace process, including paramilitary disarmament and security normalisation, although sectarianism and segregation remain major social problems, and sporadic violence has continued.The economy of Northern Ireland was the most industrialised in Ireland at the time of Partition of Ireland, but declined as a result of the political and social turmoil of the Troubles. Its economy has grown significantly since the late 1990s. The initial growth came from the "peace dividend" and increased trade with the Republic of Ireland, continuing with a significant increase in tourism, investment and business from around the world. Unemployment in Northern Ireland peaked at 17.2% in 1986, dropping to 6.1% for June–August 2014 and down by 1.2 percentage points over the year, similar to the UK figure of 6.2%.Cultural links between Northern Ireland, the rest of Ireland, and the rest of the UK are complex, with Northern Ireland sharing both the culture of Ireland and the culture of the United Kingdom. In many sports, the island of Ireland fields a single team, with the Northern Ireland national football team being an exception to this. Northern Ireland competes separately at the Commonwealth Games, and people from Northern Ireland may compete for either Great Britain or Ireland at the Olympic Games.