Others mentioned the details (1756-DHRIO, Prosoft gateway, etc).
Once you have that, you can add this gateway as a node on your DH+ network (DH+ supports up to 64 nodes). As long as you have a unique node address, it won't impact the existing communication between the PLC and your RSView. You then can access the PLC to go online through this gateway. In fact you may be able to migrate your RSView application through this gateway if you are currently using an obsolete proprietary DH+ card.
What "Ethernet I/P" functionality is required? A PC can see access the PLC through this gateway, and can even access data via OPC / DDE.
A Logix processor can do DH+ messaging to this processor. The "Path" tab of a PLC5 Typed Read/Write Message would specify the path to the DHRIO card (even in a remote Ethernet connected rack), then the DH+ message details can handle the node number.
If you need the PLC5 to message out to another PLC, you will need to set up a "Routing table" (or have a Logix processor in the same rack as the DHRIO card) . Don't know if Prosoft supports routing tables, but the DHRIO does.
If you have a TechConnect contract
March 2017 TechConnect Genius Webinar - Messaging Across DH+ and DH485 AnswerID 1041793 Has details on how to set up these routing tables.
Only Enhanced PLC-5 processors can natively support Ethernet, and only above a certain revision can natively support Ethernet I/P, however it's not a full blown implementation like Logix, so it can't really be used for IO.