"island of an island of a peninsula.."
Yes, it's possibile to reach it!!
Cagliari airport (http://www.sogaer.it/it/)
Car rental ( http://www.rentalcars.com/it/)
From the airport you can choose to take a taxi. There are two solutions: a private transfer (65 euro/car) or collective bus (15 euro/pax), and rent a scooter, a car, or an electric bike in Carloforte when you arrive.
The trip is about 45 minutes to arrive at Portovesme (Portoscuso) or Calasetta, where you take the ferry that lead you to the island of San Pietro (30 minutes_ 5,80 euro/pax).