Substantial proportions in the Appalachians and wherever there was significant Appalachian diaspora later will also have Welsh ancestry: now there's your ultimate forgotten ancestral grouping, later migration to Scranton, PA and some Mormons notwithstanding.
I have significant Welsh ancestry. In fact I'm almost 100% British according to my test results, mix of English, Scottish, and Welsh. (They have updated the results a few times to show Irish or various Scandinavian is also mixed in there a tiny amount, I'm guessing because Vikings in the latter case, but the other three are much larger and constant in every update.) But I didn't even know about the Welsh part prior to taking the DNA test.
Honestly those gene test have limited value, some guy from Jutland got a test result that he was 100% from East England. Different European population are related to each other in obvious and far less obvious ways, depending on the different migrations over time. As example if you back to 1000 the north Germans are indistinguishable from Danes and the South German and French very similar, after a millennium of internal migration the population have ended up far more of a blend, but that means if you have a Danish and French parent, a gene test may show you as German. In general Danish ancestry can be a lot older than people expect, because the Danish placement outside the migration route, kept the the mixing with external groups low (you see something similar with Sardinians and Dalmatians), while at the same time shifting environment create boom and burst cycles, which sent migration waves out of Denmark. The earliest documented group of these are the Teuton, Cimbrians and Ambrones, but the earlier Germanic expansion is likely also a result of these cycles.
We also have the example of Ashkenazim and Northern Italians, who seems closely related, but this is more likely a result of both groups being a result of similar mixing events between Mediterranean, Western European, Central European and Eastern European groups.
