Plasma-based intelligent reflecting surfaces (IRSs) have been recently proposed to reconfigure the radiation environment between transmitting and receiving antennas. Plasma-based IRSs rely on elements whose electromagnetic response is electronically controlled by varying the plasma density. Here, for the first time, the numerical design of an IRS is based on plasma discharges at the state-of-the-art. First, a cylindrical discharge has been realized and tested to identify realistic plasma parameters and geometries. Second, the design of a plasma-based IRS is proposed, accounting for practical constraints, such as the presence of the glass vessels needed to confine the plasma, the metal electrodes used to sustain the discharge, and the non-uniformity of the plasma parameters (e.g., density). Remarkably, at a central frequency of 10 GHz, a fractional bandwidth larger than 10% is feasible.