An irregular galaxy is a galaxy with no rotational symmetry.
Irregular galaxies get their odd shapes in many ways.
One way irregular galaxies are formed is when galaxies collide or come close to one another, and their gravitational forces interact. Another source of irregular galaxies may be very young galaxies that have not yet reached a symmetrical state. Also, in some irregular galaxies, like M82, young stars eject energetic bubbles gas, giving the galaxy a blobby look. They are probably formed as galaxies collide or have companion galaxies that influence them. Peculiar galaxies are generally smaller and optically dimmer than regular galaxies.