Magnetic measurements were performed on a Pr 1 . 8 5 Ce 0 . 1 5 CuO 4 - y single crystal with T c 22.5 K in magnetic fields H applied parallel to the tetragonal c-axis (H c). An anomaly in the magnetic hysteresis loop that is reminiscent of a peak effect is observed. The anomaly occurs at a nearly temperature independent crossover field H c r = 230 Oe in the reduced temperature t = T/T c region 0.75 ≤ t ≤ 0.9 and completely disappears for t ≤ 0.75. The irreversibility line follows a power law of the form H i r r (T) (1-t) 2 in the regime 0.1 ≤ t ≤ 0.9. The lower critical magnetic field H c l , c (T) was estimated from the first penetration of flux at the border between the Meissner state and the mixed state. A quantitative low temperature magnetic phase diagram based upon these measurements is presented.