You cant use IBus with SCIM installed on your computer. If you have upgraded to Ubuntu 9.10 from a previous version, or if you happened to accidentally install SCIM on Ubuntu 9.10, you have to remove it first to activate Sinhala Unicode on Ubuntu 9.10. Use following command to remove SCIM.
sudo apt-get remove scim
Go to System -> Preferences -> IBus Preferences and open "IBus Preferences" dialog box.
Go to "Input Method" tab and select a preferred keyboard layout under "Sinhala;Sinhalese" from "Select an input method" drop-down menu.
Goto your home directory (~) and open the file named ".bashrc" and add following lines to the bottom of it.
Go to System -> Preferences -> Startup Applications and open the dialog box called "Startup Applications", and click "Add" button. In the next dialog box, enter "ibus-daemon" in "Command:" text box, enter "IBus" in "Name:" text box.
Ubuntu's default Sinhala Unicode font should be replaced for a better performance. Go to /usr/share/fonts/truetype as root and delete "freefonts" directory. Install new Sinhala Unicode font and update Ubuntu's font cache.
sudo fc-cache -f -v
With a re-login, you are good to go