Gregory-

I agree with you, maybe we can call it a north Britain / Irish Sea culture area.

Regarding the African-American element, there are further connections in blackface minstrelsy and early 20th century string bands. A lot of the "plantation melodies" employed by Foster, Emmett and others were derived or at least influenced by Scottish and Irish tunes, and minstrel performers as well as working class audience members often shared Irish ancestry. No one seems to know the degree to which minstrelsy actually borrowed from African-American music, but minstrel songs did make into the repertoires of black and white musicians, and stylistic elements attributable to black musicians are a big part of the specifically "American" sound of Bluegrass.