A recently proposed evaluation of irreversible entropy changes with the help of model stochastic processes is applied to the demagnetization of an isolated square Ising lattice. A stochastic process (executed with a computer) causes an initially magnetized lattice to undergo an adiabatic demagnetization, with no external work, by flipping the spin's orientation. The condition interaction energyE = const is maintained by a pairing of flips for whichδE cancel out mutually. The demagnetization rate is controlled by letting a parameterX s (which favours one orientation over the other) decrease toward zero at the desired rate; the rate ranges from an effectively reversible to the most irreversible demagnetization. The external entropy change is evaluated from the processdiscrimination, related to the transition probabilities of the actual steps. Together with the state entropy of the lattice, this enables one to find the net entropy production characterizing an irreversible process. The evaluation is achieved without recourse to thermodynamic equivalents. A thermodynamic description, in terms of an equivalent temperature for the adiabatic process, is presented separately.