"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.
Is aisteoir mé - "I am an actor"
An aisteoir é? - "Is he an actor?"
Múinteoir is ea an fear - "The man is a teacher"
Is innealtóir í - "She is an engineer"
Ní innealtóir mé! - "I am not an engineer!"
Is dochtúir míleata í - "She is a military doctor"
Is í an dochtúir í - "She is the doctor"
An feirmeoir í? Ní hea - "Is she a farmer? She is not"
Is dlíodóir é a deartháir - "Her brother is a lawyer"
Ní dlíodóir í - "She is not a lawyer"
Is póilín í - "She is a police officer"
Is póilín é Pól - "Paul is a police officer"
Póilíní atá ionaibh - "You are police officers"