In 1781 as a member of the House of Keys he was sent to London to speak on behalf of the Island. he spoke so powerfully at the House of Lords on the 13 June that year that he helped prevent a Bill proposed by the Duke of Athol to pay extra compensation to the Duke for handing the Island over to the Crown.
In the last few months of his life he was made a Deemster or stipendiary magistrate, delayed by the opposition of the Duke.
John married his cousin Catherine Finch had eight children, a daughter and seven sons, but none of them produced an heir. His children were:
- John Finch (b. 1794, d. 1885), Lawyer, unmarried
- Michael (b. 1790, d. 1883), Captain R.N. served as a midshipman under Nelson, and later earned distinction in the Crimean War. He died aged 85, and is buried at St Peter's in Thanet. He was married, but childless.
- Hugh (d. 1822), Captain R.N. He was married and had two sons. One, Charles, died in infancy. The other, George, was in the Navy, but while still a midshipman was drowned.
- Augustus died young and unmarried.
- Philip (b1793/4 d. 1814) lived a short but eventful life. As a midshipman in the Navy he was aboard HMS Shannon and was mentioned in dispatches following the Capture of USS Chesapeake. Unfortunately he was drowned in Liverpool Bay and he was buried in his uncle's vault in St James, Liverpool.
- James Mark (b. 1794, d. 1817), advocate, and took over his father's office. He died, however, just before his father, and was buried at Braddan.
- Mark James (b. 1793, d. 1812) entered the service of the East India Company (military branch) and died in India.
- Moore, A. W. (1901). "John Cosnahan". Manx Worthies. S. K. Broadbent & Company, Limited. pp. p84. Retrieved 31 January 2012.
- The ancient ordinances and statute laws of the Isle of Man. Retrieved 2012-01-31.
- Journals of the House of Lords, Volume 36. Retrieved 2012-01-31.
- "John McHutchin IoMNHASoc Vol 5 No. 4 pp385-389". Retrieved 2012-01-31.
- "IoMNHAS vol 4 no 4 p516 - Records of Cosnahan Family". Retrieved 2012-01-31.
- The Archaeological mine, antiquarian nuggets relating to Kent by A.J. Dunkin. Retrieved 2012-01-31.
- Admiral Sir P. B. V. Broke, a memoir. Retrieved 2012-01-31.