When one thinks of the major nationalist movements, one generally thinks of a movement spawning from conservative circles. However, in Ireland, Sinn Fein has always been left-wing, as was men such as James Connolly. Why is this?
I would just like to note that European Nationalism is quite often left-wing. The SNP in Scotland, Plaid Cymru in Wales, ETA in Basque, Catalan Nationalism and Kurdish Nationalism are all considered left-of-centre, though of course right wing nationalism still exists (UKIP, the EDL, the French National Front, Golden Dawn)
This is a very recent development. The republicanism of the 1790s was egalitarian but nineteenth and twentieth century Irish nationalism were coalitions of interests. While there have been leftist movements such as Michael Davitt's land reform campaign in the 1870s or Connolly's socialist republicans in 1916, the dominant model has been an alliance between small property owners and the Catholic Church espousing morally conservative and economically populist policies. For example the 1913 lockout was organised by William Martin Murphy who was a prominent nationalist. During the civil war this coalition split over the treaty and most socialists backed the anti treaty forces (but they were far from a majority of them). The post independence IRA swung between ideological positions but was significantly left wing for some periods, particularly in the early 1930s and the 1960s. In the 1960s Cathal Goulding became chief of staff and brought the organisation in a more Marxist direction. He wanted to build coalitions with working class movements rather than relying on simple armed struggle as a strategy. This was less popular with some of the more conservative members. When the IRA split in 1969 Goulding remained in charge of the Official IRA while others formed the Provisional IRA.
During the troubles the IRA found left wing argument a useful analysis and embraced republican socialism in a bid for popular support. Despite this it remained very much a fringe movement. It is important to remember that until very recently Sinn Féin had little support. Mainstream nationalist opinion was represented in Northern Ireland by the SDLP which was social democratic. In the South, the two largest parties Fianna Fáil and Fine Gael are best described as populist. The SDLPs position came from its origins in the civil rights movement and the political beliefs of its founders but it always had a substantial element of conservative Catholic support. The shift in Irish politics that has seen Sinn Féin become the largest Northern nationalist party and left wing parties poll very well in the South happened only in the last decade so it is outside our terms.