There is always one dominant pole., but how far away it is from the other determines stability. The loop has two poles and for 1:1 mirror fed by high impedance current source will have two poles well separated., but you can't take the stability for granted always with a wilson mirror.
hi
i think it is negative feedback, to stable the stable the Im3.
eg. Vg decrease Im2 decrease, corresponding with Im5 decrease,but Vgs3 increase,so Im3 increase.
then feedback induce to stable Im3