This is an interesting map.
My own ethnic group is the largest of all ethnic groups in only one place: Upper Michigan. It's probably the only place in North America where you can't speak Finnish and rely on nobody around you understanding a word of what you say (except for "perkele", perhaps) :) One of our neighbors, the Norwegians, dominate many counties near the Canadian border in Montana, North Dakota, and Minnesota.
@old-guy-photos who lives in Michigan once pointed out how his impression of the photos I post from where I live (southern Finland) is that they look an awful lot like Michigan. Based on my Google Maps Street View adventures deciduous trees dominate the landscape of southern Michigan unlike ours. But the landscape in northern Michigan is strikingly similar to ours. The only difference in the natural environment are that different species of spruce trees are prevalent. The most common spruce in northern Europe is the Norwegian or European spruce (Picea abies) whereas the most common spruce species in North America are the Black Spruce (Picea mariana) and the White Spruce (Picea glauca). These two are the most common ones particularly in Canada and the Great Lakes area in the US. When it comes to the built environment, architecture in North America is based on British architecture and it's different from the Nordic countries. As a thought experiment, if I were sedated for a couple of days, taken to America and dropped in the middle of a North American suburb I could never mistake it for any place in my home country or any other country in Europe even if there were no cars or people in sight and if all the signs had been removed.
Interestingly, the census map shows that the vast majority of US counties have Germans as the largest ethnic group. The originally largest group of European settlers, the English, are the largest ethnic group only in counties in three states in New England: Maine, New Hampshire and Vermont, and also in most counties in Utah, a number of adjacent counties of Idaho, Nevada, Wyoming. The Irish are the largest group in most counties of Massachusetts. There are counties along the Canadian border in northern New England where the French and the largest group. I remember reading about a lot people from Quebec moving to the US from Canada a long time ago.
The border regions close to Mexico have a lot of people who identify as Mexican. No surprise there as this part of the US was conquered and annexed by the US in the Mexican-American war (1846-1848). Before that, Texas had been an independent country for about ten years since the Texas Revolution against the Mexican Central government led by colonists from the US. The president of Texas was Sam Houston after whom the largest city in the state is named. African Americans are the largest group in a very large number of counties in the South. There are also a lot of people who identify as American in that region which probably means that these people are unable to identify as predominantly having any particular ancestry.
A question for Americans, has the largest ethnic group in the county you live in had a clearly visible impact on the local culture? For example, I've been told that St. Patrick's day is big in Massachusetts and it's a great excuse for everyone who wants to get shitfaced. Is it like that everywhere in the US? Or predominantly only in the areas where Irish influence is heavy? :)