There's a great little website called ePodunk.com that has all sorts of great info. While browsing around there this morning I found a map of locations with the highest concentrations of those who describe their ancestry as Scots-Irish. In the 2000 census, 4,319,232 people in the U.S. claimed Scotch-Irish ancestry.
ePodunk mapped the top communities by percentage of population. The map and the accompanying list show communities in which 1,000 or more people listed an ancestry group, and in which at least 7 percent of those people said they were Scotch-Irish.
The link is: http://www.epodunk.com/ancestry/Scotch-Irish.html