Irregular galaxies are often chaotic in shape, with no central bulge or spiral arms. Although they used to have a more familiar shape, a dramatic collision with another galaxy has distorted their shape.
Astronomers maintain two classifications of irregular galaxies. Irr-I galaxies have some structure, but they’re still distorted enough that they can’t be classified as spiral, elliptical or lenticular shaped. Irr-II galaxies don’t have any structure at all.