I thought you was the more respectful form of thou in singular, and “ye” was the plural form of that. In Ireland you still hear ye used for plural you sometimes.

Originally the difference between ġē→ye→ye and ēow→yow→you wasn't one of number, but of case: you'd use "ye" for the subject, "you" for the subject, regardless of being a plural or a respectful singular. Much like "I" vs. "me". Eventually however "you" displaced "ye" even for the subject role, in most varieties; what you see in Hiberno English is also an innovation, but a different one.