I'm aware of slash commands. If it's a /sarcasm command, why would it be at the end of the statement?
What's your source for this? I'm pretty sure "/s" means "end of sarcasm", borrowed from XML/HTML.

You know, people tend to use sarcasm more in chats than in XML documents.

Speak for yourself

I don't remember ever seeing /s on IRC.
It's always meant </sarcasm> to me.