He was credited with resolving various border disputes in Holland, Germany and Italy at the Congress of Vienna, 1814–1815, and in his role as Ambassador to the Netherlands. For his service as ambassador to The Hague, he was awarded the hereditary title of Marquess of Heusden in the peerage of The Netherlands on 8 July 1815 by William I of the Netherlands, following the defeat of Napoleon in Brabant, in that same province's southern reaches. Trench was elected one of the 28 representative peers of Ireland on 16 December 1808. His seat in the House of Lords became hereditary when he was created Baron Trench (4 August 1815) and Viscount Clancarty (created 8 December 1823), in the Peerage of the United Kingdom, his older peerages being Irish peerages. He was a Commissioner for the Affairs of India and Custos Rotulorum of County Galway.