Mmm somebody might be able to clarify this however I don't think there is a direct bus route to/from the airport from Portinax, I think you would have to get a bus to Eivissa (Ibiza Town) then get another bus from there to Portinax, I am not even sure that you can get a bus that late in the evening?

You could look at booking a shuttle bus for approx £30 return per person through a transfer company? try these for a start but do search for others as you might find a better price.

Just found this one which is a bit cheaper http://www.holidaytaxis.com/

Graham

Ahhhh, Portinatx....

My very first "no parents" holiday was to Ibiza with the girls (4 of us) and we stayed at a small bay just across from Ibiza town. We had a brilliant fortnight but needed another holiday when we came home as generally we were arriving back from a night out in San An at 07.30am each morning...
But, I fell in love with the beach at Portinatx.... tiny bay in between two rocky cliffs.
Now I really must root out my old photos!