Ridesharing is cheap in Portugal relative to other Western European countries or the US. Bolt is usually cheaper than Uber. A Bolt ride to center of Lisbon, the Lisbon airport, or Sintra should run between 18€ to 28€. A ride to Boca do Inferno should be about 12€. Unless you plan to travel to places other than Sintra and Lisbon, you will probably save money and stress by not renting a car. And if you plan to go North or South of the Lisbon metro area before or after your stay at Casa Rosa, you may consider returning the car before checking in, or renting it afterwards. Cascais does have a few car rental locations. Having said that, there is convenient parking near Casa Rosa with prices varying on the season.
Casa Rosa is located on a semi-pedestrian/semi-restricted road, but cars are allowed to drive through, especially when dropping passengers or unloading.
From/To Lisbon:
Airport: If you plan to take an Uber or Bolt from the Lisbon airport keep in mind that the pickup area is in a parking lot upstairs just outside of the Departures section/floor, not the Arrivals floor which is what you would see in many other airports. This area is named Kiss&Fly (and often shown as such in car sharing apps).
Also keep in mind that even if you land on Terminal 2, you will be driven to Terminal 1 by bus. So, even if you landed on Terminal 2, you might will be on Terminal 1 by the time you get out on the streets.
Train: Unfortunately the train stations that connect other cities in Portugal are not directly connected to the Cascais train line. The train line from Lisbon to Cascais has no connection to other train stations, but does connect to the Lisbon subway (at Cais Do Sodre). From the airport, you can take the subway (after 1 connection) to Cais Do Sodre station. And you will be in Cascais in approximate 90 minutes (barring any strikes or delays).
While the train may not be the most convenient way to come from another city, it is convenient to go for a day trip to the center of Lisbon. You will pass next to the water for most of the trip and through Estoril (checkout the Casino from the window).
From/To Porto:
If you come by train to Lisbon (ie. from Porto), you will have to exit at Santa Apolonia station and walk for about 30 minutes to Cais De Sodre station (or take a taxi) if you wish to take the train to Cascais. A car ride from Santa Apolonia to Casa Rosa should be under 50 minutes and cost 25 € -30 €.
An easier option is to take the direct bus (once a day off season and twice a day in high season) from Porto to Cascais with FlixBus. Flixbus stops at Alcabideche which is not the center of Cascais, but closer than Lisbon (a 17 minute taxi ride).
From/To Sintra:
The only reasonable way to Sintra without a car is via ridesharing. We recommend visiting Sintra in the morning, less tourists and the palaces close early afternoon (depending on the season). Check out this website before going.