While testing the fix for JENKINS-55538 I had some queued tasks with very long descriptions (list of all nodes not having the requested label pattern).
The ngircd server drops connections where messages exceed the limit of much less than the expected by protocol 510 chars (message apparently includes the "user text", possibly expanded with utf-8?, as well as target channel/account name and other metadata). As a result, Jenkins loses the IRC connection and takes several attempts to reconnect (possibly pushing the message again and failing again).
I am not sure what can be done well about this, but one direction to look at could be a (user-configurable?) line length limit after which the message would be split (preferably on a word boundary near that limit). The ircbot-plugin already has a similar splitter to make messages with embedded newline characters into multiple single-line messages.