I already have written how it the keyboard can be installed in Ubuntu 11. However, it looks like m17n-contrib has been removed in Ubuntu 24.04, and its functionality is now included in m17n-db.
sudo apt update
sudo apt install fonts-lklug-sinhala ibus im-config ibus-m17n m17n-db
then,
Open Settings > Region & Language > Input Sources.
Click "+" (Add Input Source) and search for Sinhala or Tamil.
Select an m17n-based layout.
