Sorry for a second lengthy post, but I've had a second thought only tangentially related to my earlier one.
I think it's important here to consider emigration/immigration trends.
Here's the Wikipedia article on immigration to the USA IOTL:
https://en.wikipedia.org/wiki/History_of_immigration_to_the_United_States#1790_to_1849
There are a couple of things to consider. The first is that TTL's USA is now broken up into ~4/5 states. It's been in almost constant civil strife, suffering secessions, civil wars, and defeat to the British, for around 50 years. ITTL it's going to have a reputation for being a mess and probably won't be seen as an attractive destination for emigrants. Additionally, it's lost or is losing a lot of the large tracts of land that attracted immigrants. Next, it's got a more powerful and, perhaps more importantly, more Catholic, Mexican Empire on its doorstep.
As immigration picks up, I imagine that Mexico will be seen as the far more attractive option. This will be especially true for Catholic Irish and later Catholic Italian emigrants. We already know that ITTL there seems to be a friendly relationship between Great Britain and the Mexican Empire, so it might even be that the two powers cooperate on sending the Catholics the British don't want to Mexico. And it would be in Mexico's interest to take these immigrants because guess what the Mexicans need? Railroads to connect their northern territories to Mexico City. And guess what the Irish are renowned for building? Railroads. Guess what else the Mexicans might want? A canal in Panama! And guess what else the Irish have a reputation for building? Canals! It's also important to note that the Irish around this period are NOT anglophones:
"It is believed that Irish remained the majority tongue as late as 1800 but became a minority language during the 19th century. It is an important part of Irish nationalist identity, marking a cultural distance between Irish people and the English.
A combination of the introduction of state funded, though predominantly denominationally Church delivered, primary education (the 'National Schools'), from 1831, in which Irish was omitted from the curriculum till 1878, and only then added as a curiosity, to be learnt after English, Latin, Greek and French, and in the absence of an authorised Irish Catholic bible (An Biobla Naofa) before 1981, resulting in instruction primarily in English, or Latin. The National Schools run by the Roman Catholic Church discouraged its use until about 1890.
The Great Famine (
An Gorta Mór) hit a disproportionately high number of Irish speakers (who lived in the poorer areas heavily hit by famine deaths and emigration), translated into its rapid decline." (
https://en.wikipedia.org/wiki/History_of_the_Irish_language)
Any Irish emigrating to Mexico will just learn Spanish as they did English. I also expect that, whilst the new North American states will also want railroads, the sheer greater expanse of Mexico will mean there are far more job opportunities there.
IOTL, Germans went to the Mid West and Texas (see above link to Wikipedia) and I don't really see why this would change ITTL. Again though, Germans are not anglophones and so I don't imagine they would cause any issues in Texas as the anglophones did IOTL. I really think Texas will be fine ITTL.
I also think California will be fine. It looks like there'll be a Native Mishigama and Francophone Louisiana between California and the US nations, so I doubt we'll see an anglophone majority there. The issues regarding language shouldn't matter as much ITTL.
Mexico didn't lose California until 1848 IOTL. One of the causes of the Mexican-American War was Mexican political instability. This seems like it won't be as much a problem ITTL. A second cause was US expansionism. This is likely completely dead in the water ITTL. A third was limited Mexican control over the territory. Again, with a stronger Mexican Empire and easier Irish-built railroads making travel between Mexico City and California easier... well, that's another problem sorted.
Later, this more powerful, more economically prosperous, politically stable Mexico will probably continue to attract migrants. Especially Catholic ones from Italy that would go to New York and Argentina IOTL.
I suppose what I'm saying is that immigration was one of the key ingredients to making OTL's USA the power it is today.
ITTL, I see the butterfly effect driving a lot of that emigration/immigration towards the Mexican Empire. US political instability coupled with British-Mexican cooperation will mean Irish emigrants end up in Veracruz. Veracruz will develop into a prosperous port city and railroads will be built by Irish immigrants between Veracruz and Mexico City. Mexico City and Veracruz will begin industrialising and this will attract further immigrants. Family ties will encourage this even more so. Railroads will then be built between Mexico City and Texas. German immigrants will set up farms in Texas. Railroads towards the Pacific will be built and these will then be extended up to the Californian coast. When gold is discovered in California, migration from Mexico City is easily facilitated by the Mexican railroads - migration from the US's Eastern seaboard is much more complex, with any migrants having to first land in an anglophone nation, then cross Louisianan or Mishigaman territory, then British, then finally Mexican territory - much easier to just land at Veracruz and get the train direct. Mexico City and the Mexican cities in California will grow wealthy and industrialise. Italian immigrants will view stable, wealthy, Catholic Mexico as a much more attractive destination than one of the anglophone nations, especially given that both the climate of Mexico and the Spanish language will seem more familiar to them. Content with their northern territories being secure, the Mexican Empire will look southward; their railroads facilitating trade between the Pacific and the Atlantic are good, but a canal in Panama would be better. It would also be grand to be able to move the Imperial Mexican Navy between the Gulf of Mexico and the Pacific without having to sail all the way around South America. Any independent Central American states are encouraged, coopted, and coerced into the Mexican Empire, and the dependents of those first Irish emigrants are called upon for a new project.
The Mexican Melting Pot.
Northstar