He was the Chief Justice of Ireland from 2004–2011, having been a member of the Supreme Court since 1999. Since his term of office as Chief Justice expired on 22 July 2011 he is now the senior ordinary judge on the Supreme Court.
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William Howard Taft was elected the 27th President of the United States (1909-1913) and later became the tenth Chief Justice of the United States (1921-1930), the only person to have served in both of these offices.
https://www.whitehouse.gov/1600/presidents/williamhowardtaft
Upon the founding of the Irish Free State, he was appointed the first President of the High Court in 1924, a position he held until the death of Hugh Kennedy in December 1936, when he was appointed Chief Justice of Ireland. He was Chief Justice until 1946, when he was succeeded by Conor Maguire.
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George Washington was the only President to appoint three Chief Justices to the US Supreme Court: John Jay................1789 - 1795John Rutledge........1795 - 1795Oliver El … lsworth......1796 - 1800.
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No federal official can hold two positions at the same time. The President can become Chief Justice of the US Supreme Court after his (or her) administration has ended, or can … become President after serving as Chief Justice. William Howard Taft is an example of a President who later became Chief Justice.
http://www.answers.com/Q/What_US_President_later_became_Chief_Justice_of_the_Supreme_Court
Taft, free of the Presidency, served as Professor of Law at Yale until President Harding made him Chief Justice of the United States, a position he held until just before his death in 1930. To Taft, the appointment was his greatest honor; he wrote: I don't remember that I ever was President.. The Presidential biographies on WhiteHouse.gov are from “The Presidents of the United States of America,” by Frank Freidel and Hugh Sidey. Copyright 2006 by the White House Historical Association.
https://www.whitehouse.gov/1600/presidents/williamhowardtaft
William Howard Taft is the only person in US History to serve as both President of the United States and and a member of the US Supreme Court. In 1921, President Warren G. Harding nominated Taft to replace Chief Justice Edward Douglas White, who had died in office. The Senate confirmed his nomination in a secret vote by a margin of 60-4. Taft presided over the Court until 1930, when he was forced to retire due to ill health.
http://www.answers.com/Q/What_US_President_later_became_Chief_Justice_of_the_Supreme_Court
His most notable case as Chief Justice was the upholding by the Supreme Court of the Offences Against the State (Amendment) Bill, 1940, which proposed to legalise preventive, indeterminate internment in response to the problem of the then illegal and self-styled Irish Republican Army (IRA).
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But Taft much preferred law to politics. He was appointed a Federal circuit judge at 34. He aspired to be a member of the Supreme Court, but his wife, Helen Herron Taft, held other ambitions for him. His route to the White House was via administrative posts. President McKinley sent him to the Philippines in 1900 as chief civil administrator.
https://www.whitehouse.gov/1600/presidents/williamhowardtaft
From 1954 to 1957, he was a TD for Fine Gael for the constituency of Dublin Central. In 1971, he was appointed a Judge of the High Court and was appointed President of the High Court in 1974. He became Chief Justice in 1985.
http://www.supremecourt.ie/SupremeCourt/sclibrary3.nsf/pagecurrent/B8E6388E466B6FB680257315005A41D3?opendocument&l=en
which us president was also chief justice of the supreme court
William Howard Taft is the president of United States, was also chief justice of the supreme court.  William Howard Taft was also chief justice of the supreme court.