The Late Drew Donaldson had a very high opinion of the 500s, a view shared by others, so yes, you are right that more of them would have been good, but I'm pretty sure that he wouldn't have agreed with you about the 800s!
They had the misfortune to be built just as war loomed (of course, there should never have been a war at all, but that's another issue) so never had a chance to show how good they were (or not). The Blessed Oliver put paid to any future they might have had by ordering the A Class in the mid-fifties.
Of course, for a country with little steam-able coal, they were a nonesense as even in 1939 the yanks were proving that diesels offered a future?
Now, having acted as an Agent provocateur, let me make another suggestion.
Perhaps Ireland should have looked at electric traction? There's a long and well reasoned article in IRRS Journal Volume 1 on railway electrification - it concluded that it wasn't on then (1947/8) as it would have wolfed up masses of capital and a high proportion of the electricity then generated. It DID touch on battery power, which Dr Drumm had proved was workable and touched on partial electrification (like the "power islands" they're talking about today, to zap power into battery trains and which we will soon see - seventy years later).
Another what if?