Had Ireland not been divided, two scenarios present themselves.
First, we remain under British rule, in which case any or all of the above could have happened. One thing though - either Beeching or Thatcher would have seen off much mileage, given that we have one twelfth the population of our neighbouring island, spread out over up to half its area.
Blue and grey would certainly have trod through Limerick Junction and Portadown, probably Omagh too. And I'd say the GWR and LMS would have taken in all but the Midland Great Western and a few minor lines. I suggest the DSER and BCDR might have remained on their own too until BR.
The other scenario is that the provisions of the Home Rule Bill were carried out in full, with Ireland becoming entirely independent in 1921.
In that case, a GSR style amalgamation of all companies would give been all but certain, with the NCC, CDRJC, BCDR and GNR entering GSR / CIE ownership.
Donegal tank engines, GNR 171 and 85 in unlined grey just wouldn't be the same!