Private messages should create another window and highlight it in a different color.
By default, these are the following colors and their meanings in IRSSI:
- White - Part/Quit/Join
- Bold White - Unread channel messages
- Bold red/pink - Unread channel highlights or private messages
[Act: 2,3,4,5] text? Those numbers represent unread notifications.
2 is a private message,
3 is a ping in a channel,
4 is an unread message, and
5 is someone joining or leaving.
You can access them by cycling through with Ctrl+N or Ctrl+P, or by typing
/window N, where
N is the window number.