On a user's profile page, they should have the option to display a 'message me!' widget that allows people to send them IRC pms. The person PMing the user will PM them via the widget on the user's profile page (or their local IRC client.)

The user will receive the messages through a messaging area, we aren't sure of what to call it or what icon to use, but it'll have a basic list of the people who sent messages on the left hand side as tabs, click on a person's name and you'll get the scrollback of what they have sent to you.

PMs should also be a notification item in the user's personal feed per session (not for every PM line sent, just the first one in a given convo / session.)