Irregular Galaxyとは何か？
An irregular galaxy has no disc, no bulge and no symmetry — just knots of stars and gas scattered without any organising structure. They are usually small, usually rich in gas, and often forming stars far more vigorously than their size suggests they should.
Two things make a galaxy look like this. Some never had time or mass enough to settle: gravity organises a galaxy into a disc slowly, and a small, low-density system can still be in the middle of that process after billions of years. Others were organised once and got wrecked — pulled out of shape by a larger neighbour, which strips gas out into streamers and triggers star formation as it compresses what is left.
The nearest examples are visible to the naked eye from the southern hemisphere. The Large and Small Magellanic Clouds are satellites of the Milky Way, being steadily disrupted by it, and the Large Cloud contains the Tarantula Nebula — the most active star-forming region in the entire Local Group, far outshining anything in our own much larger galaxy. Being small and disorganised does not make a galaxy quiet.
They also matter out of proportion to their size, because they are the closest thing available to the early universe. The first galaxies were all small, clumpy and gas-rich; irregulars have low abundances of heavy elements and are still building the same way. The model reflects the structure rather than smoothing it: instead of orbits about a single nucleus, stars here orbit inside scattered clumps, which is why there is no centre to the object at all.