Sorry, I got pissed off in an arguement and decided to post this.
The main reason for the lack of a united Ireland is down to the mutual dislike between Presbyterians and Catholics. This initally goes back to the Ulster Plantations where the native Irish resistance to the Scottish planters was incredibly strong and particularly bloody due to the mutual religious hatred on both sides. However, before the Act of Union 1801 and the repeal of the last of the Penal Laws, both Catholics and Presbyterians were discriminated against, with the majority of control in Ireland resting in the hands of the landed gentry, nearly all of them members of the Church of England. They were known as the Protestant Ascendancy.
It is because of this that the United Irishmen were formed in 1791, under the belief that the Protestant Ascendancy had no intrest in the welfare of all the people of Ireland. Whilst mainly Presbyterian in origin, the movement was under the influence of the American and French revolutions and looked for a liberal Irish republic free of religious persecution under which all Irishmen could prosper.
After they're failed rebellion and the Act of Union, the final Penal Laws were repealed for Presbyterians but remained in place for Catholics. From this point, the divide seen today in modern Ireland becomes clear. The cause for Irish autonomy from Great Britain becomes largely Catholic dominated, with the Presbyterians of Ulster becoming fiercely in favour of Union. As the 19th century wore on, relations between the two groups became increasingly fraught, particularly in Ulster where the Catholic minority was discriminated against to varying degrees.
Tensions reached boiling point in 1912, when the Irish Parliamentary party looked to be ready to achieve Home Rule. The Ulster Unionist leadership approved the creation of the Ulster Volunteers and signed the Ulster Convenant, effectively declaring that should Home be implemented in Ireland they would openly rebel against both the Dublin government and the United Kingdom and establish their own state in Ulster. The hypocricy of this move simply goes to show the commitment of Unionists at the time to opposing Home Rule.
So, how does one go about achieving a united and independent Ireland? Very simply, Socialism. I'm honestly suprised I never thought of it before really, with my history of advocating radical Socialism/Marxism I should have done it a long time ago.
My POD is in 1882, where Michael Davitt, a leading figure in the Irish National Land League, does not agree to drop his more radical programme of land nationalisation for Charles Stewart Parnell. This can lead to a version of history where Socialism has a much stronger presence in Ireland, both in poor rural areas and in the more industrialized north. IOTL, Unionism was always highly reactionary, and as yet I've failed to work out why it had more support than more left-wing ideologies. My guess however is simply an appeal to British nationalism, secterian fear mongering and the same reforms carried out in Britain that prevented it from falling to left-wing revolt IOTL.
A dedicated, pan-Irish labour movement that attempts to appeal to both rural and industrial workers has some chance of overcoming the religious divide that prevented Irish unity IOTL. Whilst it may not be effective in actually leading to direct action being taken against Britain, it may well lead to significant social change and negate secterian tensions to some extent when dealing with the case of Irish Home Rule.
So, what does everyone think?