Firstly, when De Valera gave up the fight against the Free State government and decided to enter constitutional politics (1926) he was abandoning his old Republican principles. He had to take the oath to the King of England and recognise Ireland's Dominion status with the British Empire (as against asserting that Ireland was an independent republic), among other things. When De Valera was successful at the polls and formed a government around 1931 the Republican hard-liners (his ex-comrades) continued their fight "for the Republic" in the form of Sinn Fein and the IRA--which declared their allegiance to the First Dáil (of 1919, I think).

The IRA was declared an illegal organisation, members were imprisoned during the 1930's and during and after WW2 they were interned for several years. Several IRA members were executed for various offences involving guns and bombs during this period.

Hard-line Republicans like Dominic Behan considered De Valera a traitor for giving up the fight back in the 20's, especially in the light of so many of his own ex-comrades having been executed, sometimes in a brutal manner, by the Free Staters, and then turning against his ex-comrades in the 30's and 40's. Dominic's brother Brendan was imprisoned twice (once in England and later in Dublin) for his Republican activities.   

Don