Robert Stewart, Viscount Castlereagh and 2nd Marquess of Londonderry (1769-1822). Better known as Viscount Castlereagh. As Chief Secretary for Ireland (1798-1801) was instrumental in the passage of the Act of Union in 1800, but his attempt to achieve subsequent political emancipation of Roman Catholics was unsuccessful. Pictured with ass’s ears and the motto Eiren-go-bray [Ireland Forever].
From Don Juan Asmodeus, A Political Lecture on Heads, alias Blockheads!! A Characteristic Poem: Containing the Heads of Derry Down Triangle, the State Jackal … [etc.] Drawn from Craniological Inspection, after the Manner of Doctors Gall and Spurzheim, of Vienna (London: J. Fairburn [1818?]). Graphic Arts Collection Cruik 1815.5.
Below is the frontispiece fold out by George Cruikshank, in which Castlereagh is called Derry-Down-Triangle:
Here for further details of the book.