Cas·tle·reagh 
(kăsəl-rā′), Viscount. Title of Robert Stewart, Second Marquis of Londonderry. 1769-1822.
(kăsəl-rā′), Viscount. Title of Robert Stewart, Second Marquis of Londonderry. 1769-1822. British politician who as chief secretary for Ireland (1798-1801) was able to quell the 1798 rebellion and form a political union between Ireland and Great Britain (1800).