The Flask IRC channel is
#pocoo on irc.freenode.net.
Flask shares that channel with other Pocoo projects, so please
let the others know that you are talking about Flask when asking
You can use any IRC client to connect to freenode. If you don't have one yet, pick one from the list:
Alternatively you can also use the freenode web interface to join the IRC channel: open #pocoo in your browser.
If you want to read up on older discussions you can access the IRC logs on dev.pocoo.org/irclogs.