Lots of Welshmen came to Pennsylvania, New York and further south. Does Bryn Mawr sound Welsh, for instance>

You don't notice them because they blended into the general population. The Welsh have a limited naming stock and so many names are common and general. I don't know if Celts tend to be more clannish and therefore carry the same names or not, but if you look at the evidence, Scots Highlands, Cornwall, Man and Ireland, that seems to be the case.

A Celtic group, predominately Protestant, that is undoubtedly most prevalent in parts of the South. Thomas Jefferson was a Welshman, and undoubtedly the most famous person of that ancestry. I only met one person of this group in 30 plus years of living in New England. Jones, one of the most common names in the English speaking world, is of Welsh origin. The common name Walsh is an Irish one, but refers to an Irishman of Welsh roots from way back.