They are generally just a comglomeration of galaxies which are united only by the fact that they don't belong anywhere else. Irregular galaxies have a chaotic appearance, with large clouds of gas and dust mixed with both old and young stars at random. They have no apparent spiral arms or nuclear bulge.
Irregulars are generally faint.
Irregular galaxies are divided into two groups. Other than this classification, irregular galaxies can have pretty much any shape they please.