When a new law made candidates promise to take the oath if elected , de Valera and his TDs took the Oath of Allegiance in 1927 . De Valera began a legal case to challenge the requirement that his party take the Oath , but the assassination of the Vice-President of the Executive Council ( deputy prime minister ) Kevin O'Higgins led the Executive Council under W. T. Cosgrave to introduce a Bill requiring all DÃ il candidates to promise on oath that if they were elected they would take the Oath of Allegiance . Forced into a corner , and faced with the option of staying outside politics forever or taking the oath and entering , de Valera and his TDs took the Oath of Allegiance in 1927 , though de Valera himself described the Oath as `` an empty political formula . ''