I've done some searching myself and I don't find an RC5/RS-232 connection. It appears to me, at least at this point, there ain't no such critter!
Basically, you are talking about an InferRed Transmitter transmitting codes according to the RC5 standard. Then, after transmitting those codes, you want some device to receive those codes, and then, have that device pass the code onto the PLC over an RS-232 line. Then, you expect that the PLC will understand those codes!
Sounds simple enough! It AIN'T!
I do KNOW that this kind of action is occurring somewhere in the world (IR to RS-232)! Although, not in the case of a PLC!
I don't know of any PLC's that accept or understand RC5 Code. The only ones I've seen that accept IR code, at all, are the one's that accept a proprietary code for down-load.
I don't think you can send RC5 data directly to the PLC via RS-232 as long as the PLC does NOT know what to do with that data!
The problem is, as I see it... the PLC does NOT know what the hell RC5 is, and doesn't know it when it sees it!
The only way that I can suggest is, using RC5 with 4 code bits (serial transmitter), build a serial receiver (using Siemans SHF506-36) and send that output to a serial-decoder and then send the decoded signals to 4 PLC Inputs.
Considering the advantages, this sounds like a real good thing to include in PLC communications!