Is it possible to do this? I've got a window 'dev' with several panes in it. One of those windows is an irssi process. Is it possible to make that same irssi process appear in window 'play' as another pane?
tmux provides an interesting and useful way to link entire windows, but I have not found the ability to link individual panes. However, there is a way to go about getting panes linked by nesting a separate (non-default) tmux-socket within another.
First, in the desired pane of 'dev' start the session
I suggest using a different tmux-configuration file for this since you may want to change/remove the status-line or give it a less useful prefix to essentially disable it.
After you started the pane, go to your other window 'play' and select the desired pane. Now all you have to do is attach this pane to the other irssi socket
This will mean that anything changed in the irssi tmux session will be immediately visible in both windows. Hopefully this gets you the desired effect.
One more word of advice I will add here, this may end up linking two panes of different sizes. If this happens, it will reduce the useable area for your irssi application to the smallest region that will fit both panes. To fix this, you may want to adjust the size of the panes. You can do this at any time using tmux's resize pane functions/hotkeys.