A pretty good effort with some Pythonesque Latin
Yes, futuaris is 'you'd be fucked' (French foutou)
But if the consequence is in the subjunctive, that implies a hypothetical condition,
so the other verb should also be subjunctive, nisi rideas, 'unless (hypothetically) you were to laugh',
but I think it would be better to make it an indicative statement, Futueris nisi ... rides 'you're fucked if you don't laugh'
And (what should be) irrisus is the passive past participle of irridere, 'laughed at, mocked, teased'.
So I'd understand 'futueris nisi irrisus rides' as 'You're fucked if you don't laugh when you've been laughed at',
i.e. (roughly) 'Fuck you if you can't take a joke'.
(PS which I see Jolly's posted while I was working on this )
Checking the history of this phrase, before my time, that's evidently what Eric Idle intended it to mean