Flights to Colombo arrive at many different times. If arriving at night it is best to have arranged transport in a private shuttle. The trip is approximately 2 1/2 hours now with the new highway.
Travelling by bus from the airport to Colombo Fort and then transferring to a bus headed for Galle. Air conditioned bus is recommended as it is always hot here! There will be plenty of people to give you directions and be of help, just use your travel smarts. If things sound too good,,, probably are!