Following the recent Ireland threads I was going to start a new thread on this topic but then I discovered this one! So I thought I'd create a zombie instead!!
To follow on from the previous contributions there was no demand at all in Ulster for it's own parliament. The Unionists were opposed to home rule in any form, they effectively had it forced on them by the British Government which seemed to want to have as little to do with Ireland as it could. Unionist leaders like Carson and Craig felt that partition would deny Ulster's industries access to their biggest markets in the south and they believed that the proposed NI wasn't viable.
So let's assume partition happens as in OTL but the Northern Ireland Parliament is never created, the first implication is that NI would have a higher representation in Westminster, pre-partition NI elected something like 33 MP's to Westminster, IOTL this was reduced to 12 as most matters passed to Stormont. Without devolution there would have been a case for retaining these seats. The second implication is that instead of having 50 years as a one party state, NI gets the same changes of government that Britain does, as the Unionists still have their links to the Tories Craig and people like Jack Andrews and Basil Brooke serve as ministers in inter war Tory administrations.
The big difference will be post 1945, as mentioned above the property vote which Unionists used to help keep control of places like Derry will be abolished in the late 1940's. This may not have had an immediate impact as it's only in the last 30 years that Nationalists, especially Republicans have begun to vote in significant numbers as they saw voting as an effective acceptance of British rule, but eventually Nationalists would have won control of Derry, Fermanagh and Tyrone. In addition instead of being a "semi-detached" part of British politics, NI issues would have been discussed at Westminster. When Gerry Fitt tried to speak on NI issues at Westminster he was blocked because of "the convention," a piece of parliamentary etiquette that Westminster didn't comment on matters that were in Stormont's jurisdiction. With no Stormont this would never have existed so Fitt, and as NI would have had more MP's including more Nationalists, would have been able to raise Nationalist grievances about discrimination on the floor of the Commons. I can see the Wilson Government being very sympathetic to what eventually became the agenda of the Civil Rights Movement, the Macmillan Government may also have been.
So under this best case scenario, the grievances which led to the Civil Rights Movement and thanks to Stormont's heavy handed response The Troubles, are addressed before the situation spins out of control in 1969. You would still have had an IRA Border Campaign in 1956 and you would have had Paisley and other Unionist hardliners claiming that they were about to be sold out to Dublin. There would still be sectarian violence mainly killings and riots but people don't turn to the paramilitaries as they did in OTL so The Troubles don't erupt, at least on the scale that they did.