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:

> いんやぁ~おめでたいよねぇ~！
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question
meaning colloquial-language interjections
What does "いんやぁ" mean?
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.

![enter image description here](https://i.stack.imgur.com/7tDnr.jpg)
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answer
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)_

<https://ja.wikipedia.org/wiki/%E5%B0%BE%E9%81%93%E5%B8%82> (WikiPedia尾道市)
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answer