Until recently, getting certificates that would work for IRC purposes has been rather difficult. Since individual IRC servers are usually available via multiple hostnames, it requires very expensive certificates (e.g. someserver.example.net's IP address might also be present in the irc.example.net, us.irc.example.net, ipv6.irc.example.net, ... DNS records). Your options consisted of getting a "Subject Alternative Name" cert with all of these subdomains, or getting a (very expensive) wildcard certificate. IRC networks don't generally have much/any money to work with and this can easily start running into hundreds of dollars per year (especially if you deploy it properly and get one cert per server).

It's theoretically possible to deploy Let's Encrypt now for IRC servers, which removes the cost issue. The new problems become the automated creation and deployment of IRC server certificates. You'll most likely need to use the DNS-01 challenge type, since most IRC servers aren't running a HTTPd and even if they were, you couldn't guarantee that the ACME server would pick the IP of the actual requesting server out of the "pool" records (e.g. irc.example.net). Using DNS-01 means you'll need to write code to interface with your DNS server, which also means securing that interaction (so other people can't modify your DNS records and get signed certs for your domain as well).

I actually manage the (signed) certificates for one of the IRC networks I'm an administrator for. Our two blockers for deploying Let's Encrypt are the aforementioned challenges with DNS-01, and the fact that our software currently validates server-to-server links using the fingerprint of each server's individual certificate, hardcoded into the configuration file. If we're switching certs every 3 months, we'll need some way to either distribute certificate configuration more efficiently or we'll need to change the ircd to verify the certificate chain instead of the fingerprint.

FWIW, our current deployment of signed certs is the "one cert to rule them all" deployed to all servers on our network. This is definitely not ideal, but it's by far the cheapest and easiest option prior to Let's Encrypt. All other options we evaluated were simply way too expensive ($X,000+), extremely labor-intensive (e.g. manually obtaining a new cert for every single server every year/every time something changed), or both.

SSL certs from existing CAs can be used for anything, not just HTTPS.