Ulster loyalism is a militant Unionist ideology held mostly by Protestants in Northern Ireland. Some individuals claim that that Ulster loyalists are working class unionists willing to use violence in order to achieve their aims. However, others, such as Garrett Fitzgerald, argue that loyalism is simply "loyalty to Ulster not to the Union with Britain and it is mis-described as unionism."

Unionists support the continuation of Northern Ireland's membership in the United Kingdom and oppose joining the Republic of Ireland in a united Ireland. However, whilst some loyalist groups seek to maintain Northern Ireland's position within the UK they are not defined by this aim and some, such as the Ulster Defence Association, have openly supported the idea of Ulster independence. Unionists also believe in achieving their aims through purely constitutional means, but are willing to use violence to defend the constitutional process. However loyalists support the use of militant methods as the primary means to reject amalgamation with the Republic of Ireland. Consequently, anti-Unionists and anti-Loyalists frequently use the term loyalist to describe illegal paramilitary organisations.

Upon Irish independence in 1921 — despite a majority of Irish people desiring a united Ireland — six of the nine counties in the province of Ulster were permitted to opt out of the independent Irish Free State (later the Republic of Ireland). These counties, four out of the six having Protestant majorities, remained a part of the United Kingdom. The other two Ulster counties also remained part of the UK, despite having narrow Irish nationalist majorities. Both unionist and nationalist communities have allowed or encouraged sectarianism among Protestants (associated with unionism) and Roman Catholics (associated with nationalism) to further political aims.

The difference of opinion between Northern Ireland's Nationalist Catholic population (which mostly supports leaving the UK in favour of uniting with the Republic of Ireland) and its Protestant Unionist population (which mainly supports remaining as part of the United Kingdom) has led to a long-running bloody conflict known as The Troubles. However, the majority of people who live in the region do not support paramilitaries of any ideology.