Sometimes jerks make use of Tor to troll IRC channels.
This abuse results in IP-specific temporary bans ("klines" in IRC lingo), as the network operators try to keep the troll off of their network.
This response underscores a fundamental flaw in IRC's security model:
they assume that IP addresses equate to humans, and by banning the IP address they can ban the human.
In reality, this is not the case — many such trolls routinely make use of the literally millions of open proxies and compromised computers around the Internet.
The IRC networks are fighting a losing battle of trying to block all these nodes, and an entire cottage industry of blocklists and counter-trolls has sprung up based on this flawed security model (not unlike the antivirus industry).
The Tor network is just a drop in the bucket here.
On the other hand, from the viewpoint of IRC server operators, security is not an all-or-nothing thing.
By responding quickly to trolls or any other social attack, it may be possible to make the attack scenario less attractive to the attacker.
And most individual IP addresses do equate to individual humans, on any given IRC network at any given time.
The exceptions include NAT gateways which may be allocated access as special cases.
While it's a losing battle to try to stop the use of open proxies, it's not generally a losing battle to keep klining a single ill-behaved IRC user until that user gets bored and goes away.
But the real answer is to implement application-level auth systems, to let in well-behaving users and keep out badly-behaving users.
This needs to be based on some property of the human (such as a password they know), not some property of the way their packets are transported.
Of course, not all IRC networks are trying to ban Tor nodes.
After all, quite a few people use Tor to IRC in privacy in order to carry on legitimate communications without tying them to their real-world identity.
Each IRC network needs to decide for itself if blocking a few more of the millions of IPs that bad people can use is worth losing the contributions from the well-behaved Tor users.
If you're being blocked, have a discussion with the network operators and explain the issues to them.
They may not be aware of the existence of Tor at all, or they may not be aware that the hostnames they're klining are Tor exit nodes.
If you explain the problem, and they conclude that Tor ought to be blocked, you may want to consider moving to a network that is more open to free speech.
Maybe inviting them to #tor on irc.oftc.net will help show them that we are not all evil people.
Finally, if you become aware of an IRC network that seems to be blocking Tor, or a single Tor exit node, please put that information on The Tor IRC block tracker so that others can share.
At least one IRC network consults that page to unblock exit nodes that have been blocked inadvertently.