The cost of NI is always an issue, though again it depends on when we are talking about, the subvention funding of NI is far less in the 20's than nowadays even adjusted for inflation, also think that only started in the mid 20's so maybe not. The other major issue Dublin always has is importing the instability of NI. In this TL that would be done automatically to a degree if you had a forced movement of Catholics. Hell my Grandmother on my mothers side is a clear example, they well a fairly middle class Catholic family in Belfast that were "encouraged" to move post partition, to the day she died she never stopped hating England for Partition. Multiple that by several hundred thousand and as I've said you have an extremely hostile body politic in Ireland towards the UK with all that brings.
There is of course a parallel group among NI Protestants, descendants of RIC officers, the Dublin working class Protestants who were intimidated out, shopkeepers who were boycotted out, some of the strongest Loyalist positions have been taken by their descendants.
The Six counties were picked deliberately to create the position of a majority, not the 9 of Ulster that would have been to even, nor the 4 of total Unionist majority, not sure that "legitimacy" comes into it.
The initial plan (by Asquith's government) was to exclude the four clear Unionist majority counties and initial Unionist desire was to exclude the nine counties with substantial Protestant populations. This would have still had a 55% Unionist majority, the bulk of population was on the East Coast. Slightly more evenly distributed than today but the demographic trend was already there.
After the war, there was a desire to reward the conspicuously loyal Unionists for their immense sacrifices (they had their own Erskine Childers figure, a young English civil servant turned soldier called Wilfred Spender was so awed by the performance of the Ulster Division at the Somme that he adopted the Unionist cause and became the head of the NI Civil Service) and to punish the hostile Sinn Feiners. At the same time, Carson who was a more romantic Unionist and who had more of a desire to wreck Irish moves to national independence had lost much of his political credibility as a consequence of his poor performance as a wartime Minister ceased to hold the dominant position among the Unionists and was replaced by the much more hard headed James Craig and Milne Barbour. The latter were prepared to take on Fermanagh and Tyrone (49 and 48% Unionist respectively) but not to saddle themselves with Donegal or Monaghan with their third Unionist populations or Cavan with its quarter Unionist population. They saw themselves as having achieved a compact state with an inbuilt Unionist majority. They had of course no idea of forthcoming industrial decline which would disproportionately prompt working class Protestant emigration or the demographic consequences of the relatively reliable condom and the Pill.
They didn't want South Armagh but weren't prepared to give it up unless they got something in exchange.
The other factor that we don't properly appreciate nowadays is that Northern Nationalists have been radicalised by the years of Stormont rule and that a hundred years ago they were the most cautious and Redmondite wing of Irish nationalism. This wasn't surprising. Ulster Protestants might well have kept the cream of the jobs to their own community but there wasn't significant unemployment in either community before the mid 1920s. If Home Rule were to bring disaster and a loss of the Empire trade as Unionists prophesied then Northern Catholics/Nationalists were equally screwed. If the shipyards closed and trade died up, the unskilled Catholic workers were even more likely to starve than the Protestant riveters and engineers, the Catholic dockers than their Protestant foreman, if the Unionist businessmen went bankrupt, their Catholic tailor went down with them....
The leading IPP councillor in Belfast City Council referred to Sinn Fein as a "noxious weed" in his private diaries.
