Nationalist Parties in Northern Ireland can rely on around 40%-42% of the vote. Voters for these parties tend to consider themselves Irish, Catholic, and wish for Northern Ireland to join Ireland. While this applies to some voters for the 'other' parties, it's not common for Nationalist voters to vote for Unionist parties; though it can happen when the only parties remaining during ballot counting are Unionist in nature.
I am going to focus on 3 parties in this section. Sinn Fein, the SDLP, and PBP.
It should be noted that People Before Profit is not officially a nationalist party; however their priorities, their stances on the issues, and the voter base they draw from does put them directly in line with the other Nationalist parties, and so, I will be including them here.
In the last election, 263910 votes were cast for one of these 3 parties that I am considering Nationalist for reasons outlined above. This is what I will call "the Nationalist vote"
Sinn Fein is officially and Ireland-wide party that has a Northern Ireland branch. I've gone over the history of the party, in general, in other posts looking at the history of Northern Ireland and Ireland. The Party is strongly dedicated to Irish Unification, and takes a left-wing stance to other issues. In the European Parliament SF sits with the United Left group, which is one of the more left-wing groupings in Europe.
I've already spelled out the party's history to a great degree in my these past posts on Irish History in general, The Troubles in particular, and past NI Elections. Important to note is the modern party was founded with two main goals in mind; first to unite Ireland, and second, to be an anti-capitalist voice. Since then, while the party has softened enough on Irish unity to enable it to sit in the NI Executive, it has softened more on issues of economics, becoming simply a hard-left party rather than one that is rabidly anti-capitalist.
The party managed to take 63.2% of the Nationalist vote in the last election, and as such, has been the main voice of the Nationalist community.
Social Democratic and Labour Party
The SDLP was the first nationalist party in Northern Ireland to have true and widespread success.
The SDLP is a merger of two movements in Northern Ireland; the Nationalist movement and the Labour movement. Consider that in 1962, the two groups made up the bulk of the opposition. Keep in mind as well that most seats went uncontested at this time. It was not until 1965 that the Nationalists agreed to take up their seats in the NI Parliament, and that only would last for a few years. However, that time gave them the chance to get used to the idea of participating within the existing Parliament.
In 1970 a group of 6 MLAs formed the SDLP. Rather than abstaining, they demanded reforms to the system. When reforms came in 1973, the party became the main voice for the entire Nationalist community, with other, smaller Nationalist and Labour parties falling by the wayside. The party has maintained some limited ties to both the Irish Labour party and the UK Labour party, but in terms of policy is probably closest to Fianna Fail, a moderate-left Irish political party. The biggest problem for the SDLP in recent years has been competition with SF.
In the last election, the party took 31.6% of the nationalist vote.
People Before Profit
Like SF, PBP is an Ireland-wide party that has a Northern Ireland branch. Unlike SF, the local branch seems a much weaker idea, and the party operates as a subservient tributary of the larger party.
The party was officially founded in 2005 in Dublin by the Socialist Workers Party, which is openly Trotskyist. While the official line of the party is that it unites Catholic and Protestant workers, the party's history, though the SWP, has seen it take a vague view on the violence committed by the PIRA during the troubles.
PBP is a fairly radical far-left party that rejects Capitalism outright. When it came time to declare their affiliation in the assembly; Nationalist Unionist or Other, they declared it to be "Socialist" and so were grouped in with the "Other" camp.
The party managed 5.2% of the Nationalist vote, but that support was heavily concentrated in two constituencies, Foyle where Derry is, and Belfast West.