In an irregular meter, the time signature changes from measure to measure. This means that the number of beats in each measure can vary, leading to a less predictable rhythmic structure, which distinguishes it from regular meters where the time signature remains consistent throughout.
However, it is also true that depending on the context, the beats may vary in length or the rhythmic emphasis may fall between the beats, but the defining characteristic of irregular meter is primarily the changing time signatures. The option about the quarter-note pulse remaining constant would typically pertain to regular meters rather than irregular ones.