I have heard it said that one of the greatest crimes The British committed against The Irish was not to limit the amount of Irish that it let emigrate to it, as the States did from the 1960s onwards by the way the latter was at the Irish government's insistence!
No substantial Irish emigration (which had started to the new world in earnest in the 1820s, to Canada first!) would have had a profound effect on the world at large.
1. Ireland today would be between 15-25 million in population, with several cities approaching 1 million, Dublin @ 5 million and a significant European metropolis.
2. Ireland would be a federal independent state, the religious fanatics from North and south would have been well and truly marginalised
3. Genuine wealth would have accrued from our exploitation of forrestry and fisheries resources
4. We would have a large internal market with such a population with many prominent indigenous companies with a worldwide reputation.
5. No Irish America means a profoundly different United States, though whether that means more conservative or more radical, I don't know!
6. A different Scotland
7. perhaps no Manchester or liverpool, but a vastly different England
8. A Changed Canada, Austrailia and to an extent New Zealand and S. Africa, even Argentina!
5. The Irish language would be thriving
6. Gaelic sports would have a much wider audience, maybe hurling would be an olympic sport, certainly with a larger population base, we would have much more success in sports worldwide.
7. Genuine right - left divide in Irish politics
8. Unavoidable enforced participation in WW2
9 Ireland in Nato/Warsaw Pact, now there's one for you all!
They're just a few of the top of my head, I would welcome your views!