The Irish Unionist Alliance (IUA), also known as the Irish Unionist Party, Irish Unionists or simply the Unionists, was a unionist political party founded in Ireland in 1891 from a merger of the Irish Conservative Party and the Irish Loyal and Patriotic Union (ILPU) to oppose plans for home rule for Ireland within the United Kingdom of Great Britain and Ireland. The party was led for much of its existence by Colonel Edward James Saunderson and later by St John Brodrick, 1st Earl of Midleton. In total, eighty-six members of the House of Lords affiliated themselves with the Irish Unionist Alliance, although its broader membership among Irish voters outside Ulster was relatively small.

Unionism in Ireland is a political tradition that professes loyalty to the crown of the United Kingdom and to the union it represents with England, Scotland and Wales. The overwhelming sentiment of Ireland's Protestant minority, unionism mobilised in the decades following Catholic Emancipation in 1829 to oppose restoration of a separate Irish parliament. Since Partition in 1921, as Ulster unionism its goal has been to retain Northern Ireland as a devolved region within the United Kingdom and to resist the prospect of an all-Ireland republic. Within the framework of the 1998 Belfast Agreement, which concluded three decades of political violence, unionists have shared office with Irish nationalists in a reformed Northern Ireland Assembly. As of February 2024, they no longer do so as the larger faction: they serve in an executive with an Irish republican First Minister.