His mother was a sister of Timothy Michael Healy, the first Governor-General of the Irish Free State, and Sullivan in turn married their daughter Maeve.
His strongly nationalist background made him acceptable to the new Government of the Irish Free State as a member of the new judiciary and accordingly, in 1924, he was appointed President of the High Court; in 1936, on the death of Hugh Kennedy, he was appointed Chief Justice of Ireland and served until he reached retirement age in 1946.
His most notable judgment was the upholding by the Supreme Court in 1940 of the constitutionality of the Offences Against the State (Amendment) Bill which allowed for indefinite detention of suspected IRA members.
The claim seems to be unfounded: Sullivan was a firm believer in judicial independence, and in any case, by 1940 any political sympathies he had were with the Opposition, not the Government.
[4] His cousin Maurice Healy in his celebrated memoir The Old Munster Circuit[5] portrays Timothy as a kindly, serious young man; Mr Justice MacKenzie in his memoir Lawful Occasions[6] recalled the much older Sullivan, then Chief Justice, as "an old-fashioned Irish gentleman, quiet living".