Irregulars often look a mess. They show no trace of spiral structure or a central bulge. This can often be the result of a recent interaction with nearby galaxies.
These systems are also called peculiar. They make up around 25% of all known galaxies. In general these systems have lots of gas and dust in them. This means that they can be home to a lot of new stars being born.
The closest galaxy to the Milky Way is an irregular galaxy called the Canis Major Dwarf. It has only 1 billion stars. It is thought to be falling towards the Milky Way, and may one day become part of our spiral arms.