/notifyhelps me to at least see when they were online, but it doesn't help me catch them while they're still online. Xchat's "Friends" list doesn't pop up an alert, unfortunately. Frustratingly I often check on IRC only to see that they had been online while I was at my computer - just busy doing something else.
So I wrote an Xchat script to help me out. It's a script that performs an arbitrary command when a matching "Notify Online" event occurs. I've set it to
msgmyself, which will cause Xchat to pop up an alert, and hopefully I'll notice that while I'm working on my dark web drug marketplace or watching pr0n.
The script: http://www.bpj-code.co.za/downloads.php/xchat/blitzchat.pl?text