Goidelic and Brythonic mutual intelligibility

There's pretty much no mutual intelligibility between Brythonic and Goidelic languages, unless you're into historical linguistics, but sometimes something like "Gaeleg yr Alban/Gaeilge na hAlban" pops up and it's like yaaayyyyyy both Celtic branches understand that we're talking about Scottish Gaelic :) No idea what else is going on though.