If I have to place a request to have a meeting in the cabin owned by Cirus, which of the following statements is correct?
The meeting is in Cirus's cabin.
The meeting is in Cirus' cabin.
But having said that, if you were a writer or editor, you would be required to follow the publisher's style guide. For instance, I freelance for a firm that requires me to use The Economist Style Guide. The "guidelines" given here include the following:
1. Use 's with singular words and names ending in s, and
2. Use s' with plurals that end in s (e.g. brothers') and plural names that take a singluar verb (e.g. Reuters').