Éamon de Valera
De Valera resigned from the presidency of Sinn Féin after losing a vote to accept the Free State Constitution (if the Oath of Allegiance was dropped).  In March 1926 he formed a new party, Fianna Fáil (Soldiers of Destiny).  Fianna Fáil was successful in elections, but for over a year a it did not take its seats in the Dáil.  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.  They said the oath was "an empty formula" that is, just words they had to say, they did not have to believe them.