Answer
The surname Londonderry was adopted by the Stewart family in 1816 when Robert Stewart, the 2nd Marquess of Londonderry, was granted the title of Earl of Londonderry. The Stewart family had held the title of Marquess of Londonderry since 1796, but it was the elevation to the earldom that led to the adoption of the Londonderry surname. Prior to this, the Stewarts had been known by the surname Pitt.