World War 2 ended in a neat little package for most countries, barring the eastern block.
Ireland and England is something thats being going on for 800 years and the result of Independence for the South in the 1920's would be much more similar to World War 1 in that there was a lot of unfinished business and not a real end to the conflict, more of a stop gap solution.
The partitioning of Northern Ireland is what kept it going for so much longer, not that there wouldn't have been fighting if Northern Ireland had been given to Ireland either; as I'm sure the majority Unionist population wouldn't have been too happy leaving the empire.
Also while the rest of Europe has moved past WW2, really for much of Ireland this is the equivalent of 13 years after the end of the troubles, so I'm sure there would have been some left over problems in Europe in 1958.
You just have to give these things time.