I've tried scouring through various Japanese dictionaries with no luck.
For some context: Person A has just heard person B and C have gotten married:
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Judging from the context you provided, 「いんやぁ」 would have to be the informal pronunciation of 「いやあ」, which is an exclamation of surprise.
「ん」 quite often appears in a similar fashion for emphasis in colloquial speech.
「すんごい」 for 「すごい」,
「うんめえ」 for 「うめえ」, etc.
It's a dialect called 尾道弁, which is being widely used by people who live around 尾道市（広島県南東部）.
いんやぁ 嫌だ。(例1.いんやぁ、しとうないわぁ)/ 言いなさい。(例2.あの人に、いんやぁ)
Resource quoted: https://bisan.co.jp/%E3%81%93%E3%82%8C%E3%81%8C%E5%B0%BE%E9%81%93%E5%BC%81%E3%81%98%E3%82%83-p-onoken005/ (これが尾道弁じゃ)