subtle

When console based programs like irssi are started with a terminal like urxvt -e irssi, they usually just change the WM_NAME of the terminal and per default, subtle uses both of the WM_CLASS values for tagging.

The problem about that is, that the tagging is normally done before the terminal really starts the desired app. To avoid that all better known terminals support the -name argument which changes the instance value (first string) of the WM_CLASS: urxvt -name irssi -e irssi

Another problem is that terminals retain their class value (second string) of the WM_CLASS and may match other tags like:

1 tag "terms" do
2   match "urxvt" 
3 end

The common solution to bypass is this to use the :instance matcher, which just matches when for the specific instance value. So the easiest way to reliable match this client is a tag like this:

1 tag "irssi" do
2   match :instance => "irssi" 
3 end

Following table shows the WM_NAME and WM_CLASS output of xprop of the various combinations:

Command	WM_NAME	WM_CLASS
urxvt	urxvt	urxvt, URxvt
urxvt -e irssi	irssi	urxvt, URxvt
urxvt -name irssi	urxvt	irssi, URxvt
urxvt -name irssi -e irssi	irssi	irssi, URxvt

Please have a look at the tagging wiki page for more information.