I assume native speakers who hadn't heard an Irish accent would have to listen more carefully than usual to understand everything fully. It's all about how 'exposed' an accent is. There are many things on tv with Irish accents in the UK, and I know many Irish (and Northern Irish) people who live here so a UK native wouldn't have any trouble with the accent.
If you wish to become acquainted with the Irish accent, try searching for videos of 'Father Ted'. It was a highly popular tv show in the 90s, and is hilarious! The whole cast is Irish, and you should be able to find a subtitled version on YouTube or something.
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