What does inion mean in Gaelic?

There's no word "inion" in standard Gaelic (either Scottish Gaelic or Irish Gaelic) that carries a commonly understood meaning. It's possible:

* It's a misspelling: Could you be thinking of a similar-sounding word? Providing more context (where you encountered the word) would help.

* It's a very regional or archaic word: Some very localized dialects might have words not found in standard dictionaries.

* It's a loanword or a name: It might be a word borrowed from another language or a personal name.

Without more information, it's impossible to say what "inion" might mean in Gaelic.