They were outside the government and after the Civil War ended they weren't really a factor in parliamentary politics if I remember correctly. And the socialist dreams of some of the revolutionaries didn't go anywhere in DeValera's Ireland. The Irish government after independence was, so far as I know, rather conservative -- in an Irish way. Now the country's becoming less and less distinctly Irish.
I don't know enough about the specifics of the Scottish case. Just saying that it's unusual for a country to have two strongly left-wing parties. Maybe Argentina qualifies. Once they have to pay for things themselves, rather than expect the English to pay things may change. Then again, if there's still oil money to fund things, it would support your view.
I believe that after Irish independence, the IRA was focused on the counties in the North, which remained in the UK.
Scotland would go lock, stock and barrel.