Get the hire car company to drop you in Ayamonte.
Islantilla is about 30km from Ayamonte by road. 20km as the fly crows.
Alternatively, Damas, the regional bus service operate a service four times a day on weekdays from Islantilla to Ayamonte at the following times Weekends about 1 service a day.:
08:30
09:30
14:30
18:30
There are buses departing from Ayamonte to Faro.
Check Cubsur's site at www.algarvebus.info for the times.