Google transliteration input method supports many languages including Sinhalese. One great feature is the availability of the ‘zero width joiner’ and ‘zero width non joiner’ support with makes is so easy to type the combined characters which are otherwise hard to get right. It includes this typing keypad as well.
In addition, the transliteration web page is also available.
With the IME installed, all you need to do is to select the input method and you are off typing in Sinhalese. සිංහළෙන් බ්ලොග් ලියමු දැන්.
Download transliterating IME from http://www.google.com/ime/transliteration/