All foreigners must register their driving license at the airport to be able to drive in Sri Lanka.
Also please note that there is Zero alcohol policy in Sri Lanka. That means if you are the driver you cannot have any alcohol in your blood stream. The police can be quite painful and therefore stick to the road rules as the police have lot of powers over the drivers.
Driving in Sri Lanka is hectic and is not for the faint hearted. If you are used to Asian driving, then it is a matter of understanding how Sri Lankans tend to overtake on small roads. One of the most important things to understand is that since the roads are quite narrow, drivers who are coming in the opposite direction to you would be overtaking vehicles on their side by coming on to your side of the road. When this happens you need to slow down the vehicle so that they can pass the other vehicle without having a head on collision without you. Once you mastered this the driving becomes quite simpler.
Also you need to watch out for pedestrians as they will cross without paying too much attention to incoming cars. So for the first couple of days you need to drive carefully till you get the hang of it.