You can certainly get there by taxi, which are very plentiful, reliable and not too expensive. If you are planning to visit the Palace (the interior is very interesting and the gardens were constructed by the same landscaper who did Versailles) as well as lunch there, using a taxi is certainly justifiable economically as well as practically. We have used the underground in Lisbon as well, it is a very good means of transportation and for longer distances it is helpful. I don't know if it runs to Queluz, but would be surprised if it didn't. There is certainly good public transportation in Lisbon but in the interests of saving time which is so valuable during travel we normally use taxis (except for the colorful little trolleys that roll through the Chiado and Bairro Alto districts).
Best regards, Don in L.A.