 He inherited the castle on the death of his elder brother Matthew in 1841.He had married in 1820 his cousin Margaret, the daughter of Dowell O'Reilly of Tullymore and had a son and heir Myles O'Reilly, who fought on the continent for the Pope and was later also an MP, and an unmarried daughter Margaret.His grandson was also named William Joseph O'Reilly (born 1864) who in turn inherited the Knock Abbey Castle estate from Myles.