This may seem to be a silly question, but my inquiring mind wants to know.

I can understand almost every idiosyncrasy between spelling and pronunciation in Gàidhlig. This one baffles me.

Where is the 's' (or sh) in ort or ceart?

Or, another question is:
Why is there no 's' in the spelling of ort, cunnartach, ceart, etc.?