An tusa an turtar? = Are you the turtle? (as opposed to him being the turtle)
An é an turtur thú? = Are you the turtle? (as opposed to the tortoise)
If we were looking for a white horse and wondered if the horse on the horizon was it:
An é sin an capall bán?
However if we saw the horse on the horizon and wondered what it was, asking is it perhaps the white horse:
An é an capall bán é sin?