"You are police officers."
Translation:Is póilíní sibh.
There is a subtle difference, yes. I suggest reading this.
Also, this quote from GnaG might help:
> often by temporary or recently attained functions / professions tá mé i mo dhochtuir = I am a doctor ("in my being a doctor"), tá tú i do fhear anois = Now, you are a man ("in your manhood")
And, to give an example. I'm currently working as a teacher for a year (before I go back to school), but I don't consider myself a teacher. So I'd say Tá mé i mo mhúinteoir instead of Is múinteoir mé.