No doubt someone will confirm this but it appears that the Airport to Estoril/Cascais shuttle bus that was operated by Scottbus stopped operating on 31st December 2006.
Estoril is about 25kms from the Airport on the other side of the metro area. Unfortunately, there are no other direct transport links from the airport to the Cais de Sodre Station from where the train to Estoril leaves from. Even taking the Metro would require a taxi and at least two changes - a hassle with bags.
At minimum if you have more than one bag you should take a taxi to Cais de Sodre and then get the train from there. You will find taxis outside Estoril Station.
Hope this helps