The concept of a clustered IP is just that there is a public IP which connects to multiple backend systems. Many front-end firewall boxes perform such a duty of standing at the front of that IP and forwarding incoming connections to application servers, also often offering services such as sticky sessions that help incoming HTTP traffic to continually make it to the same box. Apache itself, Nginx, HAProxy, all these are often used as forwarding agents, a front end recieving traffic on a bound address and forwarding it to destination application services.
Mongrel2 is another such HTTP forwarding layer. It's front end speaks HTTP and it's back end (where it forwards traffic) is ZeroMQ. It feeds the traffic to whomever picks the forward traffic up. This is a push/pull egress to the app server. The app server processes the request and sends it back over a pub/sub connection.
Hacking document on Sockets: https://github.com/zedshaw/mongrel2/blob/master/docs/manual/hacking.tex#L237
Visualized use of ZeroMQ socket