The title of Marquess of Londonderry (pronounced "Lundundry") is a title in the Peerage of Ireland created in 1816 for Robert Stewart, 1st Earl of Londonderry, father of Lord Castlereagh, the Foreign Secretary at the time.
The Marquess holds the subsidiary titles of Earl of Londonderry (Irish, 1796), Earl Vane (Peerage of the United Kingdom, 1823), Viscount Castlereagh (Irish, 1795), Viscount Seaham, of Seaham in the County of Durham (UK, 1823), Baron Londonderry (Irish, 1789), and Baron Stewart, of Stewart's Court and Ballylawn in the County of Donegal (U.K. 1814). The two United Kingdom titles were created separately for Charles William Vane, who would later become the third Marquess. The remaining titles were all created for Robert Stewart, the 1st Marquess. Stewart was one of the few people to become a Marquess without inheriting any titles prior to the creation.
Between 1823 and 1999, the Marquess sat in the House of Lords as Earl Vane. For a year prior to that, he sat as Baron Stewart.