Revolutionary Todyo
Banned
I think you're quite right in arguing the religious element for the absence of a broad-based socialist or left wing party in Northern Ireland. But there was definitely a sort-of Labour movement, its just that it had a strong sectarian streak.
The Independent Orange Order (which was, IIRC founded in the house beside the one I grew up in, just outside Larne, ooi) was seen as evidence that Ulster Protestants had "revolutionary potential" - they organised Belfast dockers and carters strikes in 1907. It's a bit late, but there's a paper online that deals somewhat with this, although most of its on the Troubles - http://www.ucc.ie/en/history/scrinium/FrenettUlster.pdf
The issue is getting socialist politics ingrained in industrial Northern Ireland before this becomes the major issue. I'd say by 1900 it's much too late - you'd need a POD before the Gaelic revival imo - you need a time when the differences between Catholic and Protestant Irishman were minimal. So definitely before a United Ireland is associated with "alien" gaelry.
The Gaelic revival did have some support amongst southern Protestants IOTL, the split being caused as things became much more Catholic dominated, not to mention the idea that learning Irish should become mandatory for entry into university.
I also think that it was between 1900-1912 that the Ulster protestants started viewing themselves as seprate from the rest of the Irish people. Before that they were all Irish, some would prefer to identify themselves as British first, Irish second, but there was no doubt that everyone born on the island of Ireland was an Irishman.