I think that’s considered a dialect of English. But it certainly has a long history behind it!
Irish is special case, because many Irish already had the experience of living in an English-dominant society where Irish was a stigmatized language. Nevertheless, there was quite a bit of Irish spoken in the US in the 19th and early 20th centuries, and some Irish-language periodicals.
According to a documentary on an Gorta Mór I saw recently, in 1840 although as a percentage Irish was at its lowest yet there were actually more native Irish speakers alive than at any other point in the past. I was shocked and saddened by that fact. I knew Irish had been in decline on the island prior to the Famine so had assumed it was already sorta dying out before the Famine all but finished it off.
I’m only aware of one Irish-language periodical, from NYC I believe, An Gaodhal. While Irish is still spoken to this day in the US, I’m not aware of any community in the 19th/20th centuries that was say an Irish-language ghetto in the way other language groups were in US cities. Were there any?
I have also been led to believe that Nova Scotia and Prince Edward Island in Canada had Scottish Gaelic speaking communities in the 19th century and possibly beyond.
Most people consider Gullah to be a creole, rather than a dialect or “foreign language” so it really depends on whether or not the OP was thinking specifically of alternate origin languages (like the Amish or the Hasidic enclaves) or would include the various creoles of different communities around the country.
I will say that Gullah as a spoken language is doing quite well compared to a lot of the other creoles in America.
It’s Pennsylvania Dutch, not German. I believe it’s based off of very old German but the two languages are not interchangeable. My grandmother speaks Pennsylvania Dutch but she said she could not keep up when she went to Germany.
Almost all Amish families will speak Pennsylvania Dutch when they are together. Little kids don’t learn English until they go to school - and almost every family has a little kid around. They do learn English in Amish schools. I’ve never met an Amish person who didn’t speak both PD and English fluently.
According to Wikipedia, Pennsylvania Dutch was the language all of the German settlers but the Amish are the only ones that stuck with it (and it also came with them to Ohio).
I don’t know about PEI but Cape Breton Island in Nova Scotia had a significant Scottish Gaelic speaking community, which went into fairly rapid decline from around the 1930s-1940s. There are still a few speakers around.
Indeed, according to: http://www.gov.ns.ca/oga/aboutgaelic.asp?lang=en
That probably had more to do with accent than anything else. There are lots of accents in spoken German, even if the written form doesn’t reflect that. Pennsylvania Dutch is more recently split off from German than American English is from British English. And yeah, there are lots of areas in Britain where an American would have a hard time keeping up. But we still all speak English (except maybe some Scots 
).
Regarding the Gaelic langauges in North America:
There does not seem to have ever been a large monolingual Irish-speaking community, though there were certainly large numbers of Irish speakers. There wasn’t even a monolingual Irish-speaking large town in Ireland at the time! For some of the same reasons.
Scottish Gaelic is a different situation, and I believe there were communities with Gaelic as a first language in North Caroline in the 18th century and in a few other places, largely as a result of the Clearances where whole communities were forced to move at once. In Ireland, people were gradually starved out, so they moved piecemeal, while at the same time English was making inroads into the Gaeltacht.
There are people who know a lot more about this subject than me, though, so take this for what it’s worth (not a ton).
I think that the completion of Interstate 10 had a lot to do with the German and Czech communities in Central Texas no longer requiring that grade school students learn those languages. Also, as has already been said, there are a lot of communities around here where people speak Spanish, just as their families did when the land was owned by Spain.
And that’s an interesting thought about entire communities moving. I believe that the Amish had a similar history with large communities emigrating more or less intact to the US and establishing insular communities.
If your community is bleeding members over the course of several decades and various community members pick different destinations, then there may be more pressure to assimilate, for example an Irish speaking community of 1000 where 100 moved to Halifax, 100 to Toronto, 300 to Boston, 300 to New York, and 200 to Washington, DC.
Some guesstimate that there are 22 languages spoken in Alaskan villages that are the primary language with English as a second language. I never met a Central Yupik who could not speak English but I did meet some Siberian Yupik speakers had difficulty with English.