I encourage this. I hope you don't mind some random comments. The Irish Party was shattered by the pro-independence force, Sinn Féin, in the 1918 general election. If that hadn't happened, the Irish Party would have won 75-80 seats, Unionists 21-25 and Labour 0-7 out of 105. This was the basic split in Irish politics before 1916, whereas the equivalent Liberal/Conservative/Labour seat split in Britain was about 165/360/75 in 1918. (http://irishpoliticalmaps.blogspot.ie/2012/05/irish-uk-general-election-1918.html) Before the Irish Party, you had a mix of home-rule activists, Whigs, and Liberals playing the same role as the majority, anti-Conservative party, with the (Anglican) Conservatives and Unionists in the minority. (http://irishpoliticalmaps.blogspot.ie/2012/12/uk-general-election-1859-ireland.html) Pro-Catholic, anti-Conservative factions had won the majority/Catholic vote for 80 years by the time of Irish independence, while Conservatives and Unionists won the minority/Protestant vote. This is the basic party system up to 1918; what about afterwards? I don't see much reason for change after 1918, except that Labour might grow at the expense of the other two parties, until the Great Depression, when all bets are off. As you say, Dublin and Belfast (and Cork) might favour Labour in poor areas. They wouldn't have had much chance elsewhere, with voters under the influence of the Catholic Church, or anti-Labour farmers or professionals. The Conservatives would do well exactly where they did up to the 1970s: north-east Ireland, fighting against a Labour minority in most areas. This extends to the Protestant-majority parts of Ulster. The Conservatives were Unionists so I don't think they would have been pro-devolution in Southern Ireland at all. They represented the Protestant religious interest in Ireland. In contrast, Irish Catholics were not usually Conservative unless they had landed estates. Most of Ireland would have been agricultural, religious-minority, and peripheral, like west Wales or the Highlands, which had a strong Liberal presence up to the Second World War. So I think the Irish Party would either act independently, or as the Irish wing of the Liberals. This could give UKGBI a lasting three-party system: Labour; Conservative and Unionist; Liberal and Home Rule. As for history, Labour would have needed Irish MPs to support their minority governments in 1923 and 1929. Interwar politics would have been Conservatives versus everyone else. (If the Liberals and Irish merged, they could have won 10 Downing Street at the 1923 general election. Think about that...) I would put almost all the significant figures from Ireland, 1979 in the Irish Party/Liberals in this alternate history. In summary: Northern Ireland, Conservative v Labour; Southern Irish cities, Irish v Labour; Southern Irish rural areas, Irish v other Irish (maybe pro-independence?).