Irish and Scottish are very similar, but they are not mutually intelligible. It's very annoying when people use the word "Gaelic" because I never know which language they're referring to. Just say "Scottish"/"Scottish Gaelic", "Irish", or "The Gaelic languages".
Not true.
They are mutually intelligible to a high degree. Native speakers, speaking slowly and clearly can understand most of what each other are saying.
I speak some Irish and have personal experience of this.
Side note: as an Irish speaker, reading Manx Gaelic, with its Welsh/English derived spelling system feels like what I imagine having a stroke feels like.
Ha! That's a great description for how completely unsettling reading Manx is.
Are they American? Then they mean Irish.
Are they Scottish? Then they mean Scottish Gaelic.
Are they Irish? Trick question, Irish people don't use the word.
In Irish, Irish is Gaeilge.