What does Amicus est iritus mean?

Well... Nothing, really.

'Amicus' is Latin for 'friend' or 'supporting', as used in 'amicus curiae' - 'friend of the court'

'Irritus' is Latin for useless, ineffective, or something done in vain. ('iritus', with one 'r' isn't a Latin word.)

The Latin 'est', for 'is' would normally follow the noun and verb...

So, 'Amicus irritus est' might be taken to mean that a 'friend', or the friendship they offer is ineffective or worthless.