Irregular describes something that's unpredictable or uneven. If your dog has irregular spots, it means that she has random splotches of color scattered across her fur.
Anything that doesn't have a pattern or schedule is irregular, like the irregular hours of a store that's only open when the owner feels like it. In grammar, an irregular part of speech is one that doesn't stick to the usual rules. Irregular can also describe something doesn't meet standards, like irregular clothing that's sold at a discount. Irregular originally meant "not in conformity with Church rules."