write an irule to route (not re-direct) the traffic to an URI instead of a pool.

I see a reference to the "pool pool_name" command, is your current scenario irule-based?

To process an iRule rewrite of this kind, you need to consider a couple things. First and foremost, you still require the basic information to craft the packet: server IP and server port. 
You don't "have" to have it bound it into a pool object, but it's usually the nicer option. As Mohamed said, if the resource has a dynamic IP you can use a FQDN object and configure DNS resolution so it will always stay up-to-date. Keep in mind that this will still only be a "node" object - meaning: only server IP address is being considered.

Next point, you need to consider routing. Fix anything that must be fixed and make sure connectivity to the object is working in both directions. 
Now, focus on higher layers. If pool seems a good idea, lock the socket into that ; the only other opion I see will be scripting a "node" instruction in the iRule. 

At application layer, modify the HTTP request parameters as required. If this is a forward and not a redirect, you need to replace the HTTP::host header, eventually rewrite the HTTP::path and perform any additional change that you need. Usually, all of this is an iRule work. 
Last, check your HTTP response as well, and verify if anything needs to be fixed there as well before forwarding it back to client.