If you've gotten an IRROPS ticket because you were bumped or a connection isn't possible, it'll usually get rebooked as Y. If you call up Delta Reservations, they may or may not actually treat it as a Y for upgrade purposes, etc.

At the airport, though, my experience is 100% that they treat it as a full Y, even though it was an IRROPS.

For instance, I had a roundtrip multihop mileage award ticket from DCA to HNL. At my first connection at MSP (or was it DET?), I asked an agent to switch around my next flight, due to a too-short connection. After he was done, I asked him if he could find me a better return flight for the journey a few weeks later. He kindly found me a much better return. Both directions were reissued as a Y fare, IRROPS.

When I called Delta Reservations to see about doing a mileage upgrade for the HNL-ATL return (they had a semi-BE cabin), I was told that I couldn't, because it was an award ticket, regardless of the current booking class. However, at the airport, no one cared, and the TA and GA both allowed me to upgrade for only 7500 miles (an upgrade from a Y-fare ticket).