From September 2011 some of the Cádiz-Jerez de la Frontera trains (CERCANÍAS C-1) stop/leave from Jerez airport. Not all trains stop at the airport though, most of them terminate at Estación de Jerez in Jerez city centre.
To find out the exact train times, please check the official transport website: http://www.cmtbc.es. Click on Horarios, then Horarios por Línea and finally C-1 Cádiz-Jerez de la Frontera / Aeropuerto de Jerez. The airport trains only run about 4 times a day each way, so do make sure you check the times before travelling.
I took the 18.10 train to the airport from Cádiz and arrived at the airport with plenty of time to board my Wednesday evening Ryanair flight to London Stansted.
The train was very convenient, cheap and simple to use, the one-way ticket cost me 3,40 euros. You can buy the tickets from the ticket office or the self-service machines at the station.
So you've just arrived on your Ryan Air flight, now what do you do? Before your only option was to get a taxi that cost €11 into the centre, now there is a bus that takes you all the way to Cadiz Capital if you want it too.
Be careful about you arrival times though as the last bus that departs from there is at 21:45 Monday to Fridays and 21:30 on Saturdays, Sundays and holidays.
The first bus in the morning leaves at 6:30.
If you are travelling to Jerez from Cadiz the first bus to leaves for the Airport is at 5:15 and the last bus it at 20:00.
The First bus from Jerez centre to the airport is at 6 and the last one at night is 20:15 Mondays to Fridays and 20:45 Saturdays, Sundays and holidays.
The buses also stop in El Puerto de Santa Maria.
It costs 95 cents to go to Jerez centre and 2,85 to go to Cadiz.
Go to the link below and click on Cadiz-Jerez:
M-050 direct to Cadiz, stopping only in Jerez centre (76 minutes to arrive to Cadiz)
M-051 to El Puerto de Santa Maria or Cadiz
M-052 to El Puerto de Santa Mª, Rio San Pedro, University and Cadiz
The M-053 goes to Jerez but NOT the airport!!!!
If you have trouble understanding the web site in Spanish I would be happy to help with queries.