What does the left believe in nowadays? I'd say a redistributive economic system, an effective welfare state, the promotion of social justice and human rights across the world and the protection of the environment.
But I wouldn't include nationalism, despite its connection with human rights in many parts of the world.
The unifying factor for socialist politics is class, not territory. The left across the world tries to get states to move closer to the key principles I've outlined, either from the outside, by campaigning, or from the inside, by standing for election.
No state is intrinsically more virtuous than any other. Certainly, on a local level, there's no advantage in class terms to either remaining part of the UK or becoming part of a united Ireland.
That doesn't mean ignoring or trivialising the chosen identity of individuals. However, it does mean no political party that seeks its support from only one 'side' is capable of representing socialism to the public, which rules out the SDLP, unless it changes fundamentally.
In a less divided society most lefties would opt, pragmatically, for working within a Labour Party. Here, we have two labour parties, and neither allow their members to stand for election, which in my opinion is a vital part of being involved in politics.
The case has been made and lost in the Irish Labour Party, and is ongoing in the UK Labour Party, currently led by Harriet Harman (below).
Another option could be an electorally focused 'popular front', consisting both of political parties and individuals. The organisation would raise funds, provide organisational support for elections, support other campaigns, and, most importantly, promote debate around developing a progressive agenda. Activists would come from all traditions and would seek support from all.
Of course there would be problems. There would be disagreements about which organisations should be included, and on priorities, strategy and tactics. The national question might still cause tensions. There's also the practical issue of who will do the work.
But without progress, we'll be one of the few places in the world where the whole community can't vote for democratic socialism.