It’s normal for IRC servers to check whether the client IP connecting is an open proxy. This is because people often connect to IRC servers via open proxies to obscure their actual email address (much as spammers often exploit open proxies to send spam).
I’m not going to get into a long discussion of what an open proxy is, and why it’s bad, but basically, HTTP proxies which haven’t been properly secured can allow people to access resources without exposing their client address. In many cases, these proxies allow people to access resources other than just websites - often they allow someone to open an arbitrary TCP connection, such as to an IRC server or a mail server, “through” the proxy. Google for “open proxy” for some more information.
So my guess is that that IRC server is seeing a connection from that IP (your site’s IP) for some reason (maybe a spoofed connection), and is doing a test to make sure that your IP isn’t an open proxy. Port 80 is one of the common ports that open proxies can live on, so that’s why you’re seeing the request in your HTTP access log. The bot is checking to see if it can connect to 18.104.22.168 (your site’s IP) and open a connection back to itself; if it can, it knows it’s found an open proxy, and it will not allow a client connecting from that IP to connect to the IRC server.
Ultimately, you should usually ignore attempts like this, and not spend too much time worrying about it.
Hope that explains things a little more… does that make sense to you?