Robert Stewart (1739-1821) was the father of the famous politician Lord Castlereagh.

He was regularly promoted through the ranks of the peerage of Ireland:

• Baron Londonderry (1789)
• Viscount Castlereagh (1795)
• Earl of Londonderry (1796) - on reaching this rank, his son took Viscount Castlereagh as a courtesy title.
• Marquess of Londonderry (1816)

Reference

• LONDONDERRY, EARLS AND MARQUESSES OF, 1911 Encyclopedia Britannica; [1] (http://22.1911encyclopedia.org/L/LO/LONDONDERRY_ROBERT_STEWART_2ND_MARQUESS_OF.htm)