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
a question.
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.