He was born in Dublin, only surviving son of Rickard Deasy and Monica O"Connor. His father was a highly successful politician and barrister who was elevated to the Bench and ended his career as a judge of the Court of Appeal (Ireland). He married Dolores Hickie, daughter of Colonel James Francis Hickie of Borrisokane, County Tipperary, and Lucila Larios de Tejeda of Castile, and had three children.
He served as a British Army Captain, mostly in India, between 1888 and 1897, when he retired. Dolores was a sister of the prominent soldier and statesman Sir William Bernard Hickie. After his army service he became one of the first westerners to write a detailed account of Tibet, covering his travels between 1897 and 1899.
He also provided photographs for a book by Percy West. Church.
Later, his interests turned to Motor Cars. In 1903 he helped promote the Rochet-Schneider Company by driving a car from London to Glasgow non-stop.
He also drove a Martini car up a mountain rock railway near Montreaux, Switzerland. At this time H H P Deasy and Company, was formed to import both Rochet-Schneider and Martini cars into the United Kingdom. In 1906 The Deasy Motor Company was formed, and took over the factory formerly used by the Iden Carolina Company at Parkside, Coventry.
Deasy became increasingly frustrated after clashes with is chief designer, Edmund Lewis (formerly of Rover and Daimler) and resigned on 9 March 1908.
In 1909, John Davenport Siddeley left Wolseley to join the Deasy Motor Company, which Siddeley later built up and merged into Hawker Siddeley Group, which ultimately became part of Rolls Royce.
