I've stayed in two different La Sirena's, if it's the all inclusive one your staying in it's not on the beach, from what I can remember there was no air con but there was a fridge though, the rooms were also very small and the food wasn't up to much, the staff were horrible as well.It's part of the G.P.S hotels group. I wouldn't stay there again.
The La Sirena that is on Es Puet beach (above La Kiki Cafe) is probably the best apartments I've stayed in Ibiza, big rooms, air con, boss sea views (amazing view of sunsets) big balcony, food and service in the bar second to none and reasonably priced, mattresses on your sunbeds around the pool, which always seemed to be quiet never a battle to get a bed. Then there's the beach just across the road, used to love going over the beach in the evening around 6 stay there until around 8 then back to the room to get on it and watch the sunset. I'd recommend it to anyone and we'll be defo staying here whenever we go back to Ibiza.
I hope it's the second La Sirena you get. We actually thought we'd booked the second when we ended up in the first one in 2010, we booked it off teletext holidays and they were using photos of the second one to promote the first one.