Sinn Féin for nine years. (It merged with two other organisations to form Fine Gael in 1933.) Anti-Treaty Sinn Féin members continued to boycott the Dáil. At a special "Ard Fheis" in March 1926, de Valera proposed that elected members be allowed to take their seats in the Dáil if and when the controversial Oath of Allegiance was removed. When his motion was defeated, de Valera resigned from Sinn Féin; on 16 May 1926, he founded his own party, Fianna Fáil, which was dedicated to republicanising the Free State from within its political structures. He took most Sinn Féin TDs with him.
Éamon de Valera elected a Fellow of the Royal Society (FRS), a recognition of his lifelong interest in mathematics. He also served as a member of the Parliament of Northern Ireland (for Down from 1921 to 1929 and for South Down from 1933 to 1937), although he held to the republican policy of abstentionism and did not take his seat in Stormont. De Valera was criticised for ending up as co-owner of one of Ireland's most influential group of newspapers, Irish Press Newspapers, funded by numerous small investors who received no dividend for decades. De Valera is alleged by critics to have helped