States with the highest Irish ancestry
First, for all AncestryDNA ethnicity estimates of people born in the same state, we averaged their fractions of Irish ethnicity. Then, we found the U.S. states whose residents have the highest, and lowest, amounts of Irish ancestry.
On the map are the top five states with the highest average Irish ancestry. Massachusetts is #1, and all of the other top states are also in the Northeast.
AncestryDNA estimates its Massachusetts-born customers average 28.5% Irish genetically, which is reasonably close to my surname-based estimate of 26% (using 1940 census data).
AncestryDNA's estimates of Irish ancestry for much of the rest of the country are likely inflated, however. AncestryDNA's "Irish" cluster spills over into Scotland and Wales, and to a lesser extent even into England and France. While (in an analysis shown in the AncestryDNA white paper) 95% of Irish are placed into the "Irish" cluster, only something like 60% of British are placed into the "Great Britain" cluster (with most of the rest presumably being placed into either the "Irish" or "Europe West" clusters). AncestryDNA's estimates rely on ADMIXTURE, an allele frequency-based approach, whereas I think very large data sets and an approach that makes use of haplotype information will be needed to clearly dissect recent ancestry within Northwestern Europe.