Sinn Fein was the party that was most involved in gaining independence from UK domination for the Irish people. After the war, it kind of went underground, because it was opposed to the treaty that split Ireland and so refused to recongnize the Irish government. So, Fine Gale was the party that supported the treaty, and Fianna Fail was a faction that split away from Sinn Fein and agreed to temporarily work within the treaty system as a practical measure. Sinn Fein, which refused to take parliamentary seats when elected (a practice, abstentionism, they inherited from the Irish Nationalists who were elected to the UK parliament), soon slipped into irrelevancy in the republic itself, only recently regaining some relevance as a left-wing alternative to the current major parties as they've finally dropped the counterproductive policy of abstentionism. However, they were very important in North Ireland, where they, along with the IRA (well, after the Provisional IRA split, initially during the beginning of the Protestant/UK oppressions the IRA mostly engaged in obscure Marxist rhetoric and didn't defend the Irish people, earning the moniker "I Ran Away"), defended the Irish Catholics from Irish Protestant/UK terrorism and oppression during the troubles, and currently, as a result of their work defending the true Irish from the traitors and oppressors, they are the second of two main parties in Northern Ireland.