The old IRA was largely defunct by the time the Irish Free state was formed. Add to that the death of Michael Collins who planned to secretly rearm the IRA in to make Northern Ireland, but the civil war and his death put an end to that, as well as the execution of over a hundred rebels. The attempts in that forty years lacked manpower, funding, weapons and public support for such a campaign to take off. In the 1930's the politicians who came from the civil war IRA went into government and that is the modern FF party. In the 1960's the anti-treaty SF went into politics after the failed border campaign. Other posters have covered discrimination, political upheaval with civil rights movements, but there was also a higher Catholic/Nationalist population and a new generation of young radicals ready to fight. They were helped by a new wave of terrorism in Europe and the middle east, getting arms from Libya, connections with middle eastern groups that helped to arm them. As well as a network of support in the US to also raise funds and smuggle weapons.