You sure can do the full JAR rating in the US! Only a British one you cannot. - Remember, this is a rating not a license, so your state of license issue does not matter.
There are Irish approved British CAA examiners in Florida (2 when I was there), who can do the Irish exam and then you can add it to any license, British or German.
I believe Lufthansa does the same in Arizona...
I went down doing an FAA IR, which turned out slightly cheaper after conversion and I do believe the FAA one is more thorough, but this is just my personal opinion.
Depending on where you live in Germany, you may want to check a few schools to the east of Germany, prices are generally lower and all the tests are in English.
Good luck! IP