how do you say, I love you in singalese?

Answer 1

mama oyaata aadareyi

Since I can't type sinhalese characters, I'll include a link to a dictionary where you can find it written in sinhalese script as well:
http://www.ceylon-online.com/sinhala_page.html
(Just type in "I love you" into the box at the right hand side at the top, where it says "search for" and press enter)

Answer 2

Mamath aadareyi