Sadleir co-founded the Catholic Defence Association in 1851 and was one of the leading figures in the Independent Irish Party which held the balance of power in the House of Commons when it formed in 1852.
He resigned his ministerial position in 1854 when he was found guilty of being implicated in a plot to imprison a depositor of the Tipperary Bank because the individual in question had refused to vote for him.
His own financial affairs were ruinous, and in his efforts to solve his problems he milked the London Bank, ruined a small Newcastle upon Tyne bank, sold forged shares of the Swedish Railway Company, raised money on forged deeds, and spent rents of properties he held in receivership and money entrusted to him as a solicitor.
Unable to face the consequences, he committed suicide near Jack Straw's Tavern on Hampstead Heath on 17 February 1856 by drinking prussic acid.
[1] Charles Dickens based the character of the great financier Mr. Merdle (who goes bankrupt and commits suicide) in Little Dorrit (1857) on John Sadleir.