The Inductivist cranked the GSS to figure which American ethnic groups are spiritual and which are not, and this is what he found:
SpiritualityEthnic Group
1.92East Asians
2.78Scandanavians
2.8Italians
2.9Irish
2.9Germans
2.91Mexicans
2.95English/Welsh
3.12American Indians
3.26Blacks
3.29Scots
The Scots may be Scotch-Irish Southern/border state charismatics and evangelicals.
I'd say the nature of East Asian spirituality (Japanese in particular) probably skews the results, given the phrasing of the question.
I would guess too that there is a bimodal distribution over at least the self-identified Scandinavians, between those who are "ancestral" - Americans with Scandinavian ancestry in the distant past - on one hand, and cultural Scandinavians - people who where born in Scandinavia or whose parents were - on the other.
Here is a genetic story. My father always said we were from Tennessee and were "Scots/Irish". My brother did a geneology search and found no Scotch and no Irish; but he did find English and a lot more American Indian than we had previously known about. People who said they were "Scots/Irish" got into that habit because they did not want to advertise their Indian blood. In the 18th and 19th centuries in America people didn't brag about that. So that is why your figures for Scots/Irish are close to to the American Indian numbers.