I did hear a native Irish speaker could converse fluently with a native Scots speaker though there are some differences in the endings and some basic words. They have a common structure and sound shape. Interestingly I met a Welsh speaker in the 70's who learnt English in the Royal Navy and on a courtesy visit to a French warship found he had no difficulty in having a fluent conversation with the Breton sailors in both their native languages; Welsh and Breton. They were both much surprised.
Not sure that I fully agree on the Irish / Scots bit. The accent is very different, for a native Irish Gaelic speaker it is easier to understand written Scots Gaelic (SG) than the spoken version. Donegal Irish (NW Ireland) is closer to SG than any of the other Irish dialects. Anyway, the Scots are just an Irish tribe that left 15 or so centuries ago (Scotus= Irish in Middle Ages e.g. Johannes Duns Scotus Eriugena

)
Breton and Cornish are closer than Breton and Welsh, Manx fits in also, but all, including Gaelic, share roots e.g. a river is abhann in Irish, abona in Old English, aven in Breton, awin in Manx.
My father was a fluent Irish speaker and years ago at a funeral in the Hebrides (Barra) had difficulty in conversing in Gaelic with an old man there. He did say that the guy had no teeth was not a help!
K.