I’d say that America (and to some extent, Australia) have disproportionately more people of either Irish or Scottish descent than there are any native Scots and Irish still living in their respective countries. (That’s if it weren’t for unfavourable socioeconomic circumstances but the same can be said of Norwegian Americans to an extent.)
I’d even say they’re that responsible in shaping at least most people’s perception of Celtic culture as there might be some exceptions to the rule in some places like France to some extent when it comes to the Bretons. (The Bretons would culturally and historically be considered as the children from the marriage between the immigrant Britons and the native Gauls.)
There are still some migrations to America by the Scots and Irish today, though parsimoniously not to the same heights as it was during the 19th and early 20th centuries.