Hi @satch, and welcome to the LE community forum 
Simple: You don't.
You first get a cert.
Then you can use that cert with any software that uses certs.
How?
Well, that depends on the software using the cert.
In this case, IRC.
You will need to review the IRC software documentation on how to secure it with a cert.
[Getting a cert with certbot and nginx should be quite easy.]