For anyone looking for this,
1. sudo apt install ibus-m17n m17n-db
2. Go to Settings -> Region & Language 
3. Click on Manage Installed Languages
4. In the next dialogue , ensuring that IBus is selected for Keyboard input method system, select Install/Remove Langauges..., make sure you see Sinahala selected. If not select Sinhala and click Apply.
5. Log out and log in again
6. Go to Settings -> Region & Language. Under Input Sources, click on the plus sign ("+") , click on Sinhala, and select your preferred input method (I use Wijesekara)
හොයන කෙනෙකුට අවශ්ය වෙන්න පුලුවන් නිසා මම කරපු පියවර ටික එවනව.