"Pepper is in the soup." is grammatically correct, though syntactically strange. The main reason that it's strange is because pepper is a singular noun that represents a group of things (grains of pepper). We would more readily say "[There is] some pepper [is] in the soup.", but you can't translate this sentence in that way because that would require whatever the Irish word for "some" is. If it was something like "Tá stéig san anraith." it would make more sense to do "Steak is in the soup.", but we would still more likely say "There is steak in the soup."
"Pepper is in the soup" is a grammatically correct sentence in English, but it doesn't mean the same thing as "there is pepper in the soup" (in English). Tá piobar sa anraith can be translated either way, but unless you know someone called Pepper who likes to swim in soup, it's likely that "there is pepper in the soup" is the intended translation.
In English, “Someone named Pepper is in the soup” is certainly a possible meaning of “Pepper is in the soup”, but I disagree that “Someone named Pepper is in the soup” would be a more common meaning than “There is pepper in the soup” for “Pepper is in the soup”.
If the sentence were “Cream is in the sauce”, would your first thought be “That can’t be — Jack Bruce died last year!”? ;*)
"There is pepper in the soup" tells us something about the soup. "Pepper is in the soup" tells us something about Pepper.
Without context, "cream is in the sauce" sounds like something a learner of Enlish might put together - a syntactically correct sentence that probably doesn't quite convey what the speaker intended. It's a not a sentence that would actually be used in any version of English that I've ever encountered, except where "cream" is a proper noun. "Cream is an ingredient in the sauce", "there is cream in the sauce", "the cream is in the sauce" would all be much more natural constructions. "cream is in the sauce" sounds like someone screwed up (either the speaker, or Mr Cream).
Again, I disagree — “Pepper is in the soup” tells us something about the soup. Think of it as a response to “What’s in the soup?” if you can’t think of any other reason to use it with that meaning.
You’ve now encountered one version of English in which it’s used with that meaning, since I’d use that sentence with that meaning.
That's just an ellision of "there is pepper in the soup". (Or perhaps the ellision is actually in the question "what is there in the soup?")
"Pepper is in the soup" is an answer to the question "Who is in the soup".
I've already said that "pepper is in the soup" is a grammatically correct sentence. It's just not semantically equivalent to "there is pepper in the soup".
If you can't see that "Pepper is in the soup" is a sentence about pepper, not soup, then we'll have to agree to disagree
Since “Pepper is in the soup” is an elision of “There is pepper in the soup”, then why wouldn’t it be a statement about the soup?
As I’d noted above, “Pepper is in the soup” can refer to a “who” as well as a “what”. Given these two semantic possibilities, and given that the medium is soup, though, I still maintain that the “what” meaning would be more common than the “who” meaning, since the ingredients of soup are discussed more often than swimmers in soup are.
san is sa before a word that starts with a vowel - it's easier to say san uisce than sa uisce
tá sé ina chónaí i mbaile suimiúil - "he lives in an interesting town"
tá sé ina chónaí in áit suimiúil - "he lives in an interesting place"
tá spúnóg sa chaife - "there is a spoon in the coffee"
tá spúnóg san uisce - "there is a spoon in the water"