I'm a fluent at speaking Irish and I live for a couple of years with a fluent speaker from the Isle of Skye and we could converse with no major issues. I'd almost compare it to just two very different accents.

Another interesting factoid is there is two streams if the celtic language, P-celtic and K-celtic and by swopping the P sound from the P one with a K sound you get to the other language.

Example

4= pedwar in Welsh, cathar in Irish, swop the K with the P in pedwar and you have Kadwar which sounds remarkably like Irish and this works with a lot of the language but not really place names