John Cosnahan (1754–1819) was the first High Bailiff of Douglas, Isle of Man, and Deemster. He was the eldest son of Hugh Cosnahan and Eleanor Finch[1] (1753–1799)[whose are these dates?].
In 1781, as a member of the House of Keys,[2] he was sent to London to speak on behalf of the Island. He spoke so powerfully in the House of Lords on 13 June that year that he helped prevent a Bill proposed by the Duke of Athol[3] to pay extra compensation to the Duke for handing the Island over to the Crown.
In 1808 he became a Water Bailiff or Judge of the Admiralty Court.[1][4]
In the last few months of his life he was made a Deemster or stipendiary magistrate, although his appointment was delayed by the opposition of the Duke.