I found the below explanation on portugaldrivinglaws.com. And, this simulation by the GNR on Facebook.

The law in Portugal for driving on roundabouts is not like the law in many other countries:

• When you enter a roundabout, always give way to traffic already on the roundabout (approaching from the left).
• By law, you are required to signal when entering the roundabout AND when exiting the roundabout. When signaling to exit, turn the signal on when you pass the exit prior to the one you are taking.
• If the roundabout only has one lane, things are simple. Enter the roundabout then take the exit you want to.
• For roundabouts with more than one lane:
  • If you want to take the first exit from a roundabout, keep to the right-hand lane as you approach the roundabout. Otherwise move to the left-hand lane as you approach the roundabout then move to an inside lane on the roundabout.
  • Stay on an inside lane on the roundabout until you approach the exit you want to take. Then move to the outside lane and take the exit.
  • You are breaking the law if you are on an outside lane on a roundabout and pass an exit without taking it.
  • Be very careful with slow-moving traffic (for example, a heavy goods vehicle) driving on the outside lane of a roundabout. If you are on an inside lane and there is any doubt about taking an exit, simply go round the roundabout again.
• You may see signs reducing speed limits as you approach a roundabout. For example, from 50 km/h to 30 km/h. Respect those reduced speeds absolutely.
• The law in Portugal governing exit from roundabouts changed in 2013. On balance, this was probably not a good change and easily confuses foreign drivers. The police are active in enforcing this law here and will frequently park on or near roundabouts issuing fines to drivers who do not use a roundabout correctly.
• Whatever the speed limit is, watch other drivers either side of you carefully on a roundabout. They may cross your path – from your left or your right – unexpectedly.