The IRW Decius was a Romulan warbird that existed only inside an illusion shown to Commander Riker in 2367, in which it carried Admiral Picard and Ambassador Tomalak to the Enterprise.
The Decius appears in the Next Generation episode Future Imperfect. Riker loses consciousness on Alfa Onias III and wakes to be told that sixteen years have passed, that he is captain of the Enterprise-D and that the Federation is about to sign a peace treaty with the Romulans.
In this supposed future the Decius brings Picard, now an admiral, Deanna Troi and Tomalak to the Enterprise to complete the negotiations. Riker is unsettled to see a Romulan warbird greeted as a friend and Tomalak walking freely about the ship.
The future is a fiction, and so is the Romulan prison that seems to lie behind it. Both were made by Barash, a lonely alien child who wanted a companion.