For UK rules you need microlight differences training signed off in your logbook, which for (say) a typical Cessna pilot to fly a medium performance 3-axis microlight is likely to take a couple of hours. But you do need that training - there are enough differences to bite if you don't understand microlights.
There's a UK microlight school in Portugal, near Lagos and run by a chap called Gerry Breen (he's got a website somewhere I'm sure). He'd be able to do that for you, but this does assume the aircraft you're borrowing is UK registered. For a Portugese registered microlight the rules might be different - but it might be worth speaking to Gerry anyhow, he's a friendly chap and presumably is fully conversant with Portugese microlight regs. (Does seaplane conversions too, which are starting to get seriously tempting in the current UK weather).
P