You could try these guys:
http://www.low-cost-transfers.com/lisbon-airport/
They are quoting €54.00 for 2 people from Lisbon airport to Ericeira direct.
By the time you get a bus or taxi to Lisbon Cais de Sodre Station, then a train to Cascais or a bus or taxi to Sete Rios and then a train to Sintra, and then a taxi to Ericeira, you will probably find it better to get a direct transfer especially if you have to be in Ericeira 11:30.
The bus company Mafrense have a service from Lisbon Campo Grande to Ericeira. The September 2008 timetable shows departures on Saturday Morning as follows:
07:35 Arr: 09:04
09:20 Arr: 10:34
You would need to get a taxi from Lisbon Airport to Campo Grande.
Downloadable here:
http://www.mafrense.pt/downloads/ericeira_lisboa_setembro_2008.pdf
