Sorry, but I dont consider Sinn Féin to be a republican party, not any longer. To me, they're a constitutional nationalist party like the SDLP. Gerry Adams once rang the bell at the New York Stock Exchange - just goes to show how he feels about the working class.
Anyway, the main problem right now among Republican Socialists is that you'd get a majority vote across the entire island for a united Ireland, but for a Socialist Republic you'd get maybe 30%. Ireland will most likely be united as a capitalist state, and continue to be screwed over by the US under the term "the Celtic Tiger." Also, with the Free State government becoming more and more Thatcher-like, many people feel that reuniting Ireland is only the first step in achieveing independence.
“There are no boundaries in this struggle to the death. We cannot be indifferent to what happens anywhere in the world, for a victory by any country over imperialism is our victory; just as any country's defeat is a defeat for all of us.” – Che Guevara
“We still believe that the struggle of Ireland for freedom is a part of the world-wide upward movement of the toilers of the earth, and we still believe that the emancipation of the working class carries within it the end of all tyranny – national, political and social.” – James Connolly