Reinforced concrete staircases are structurally designed as cantilevered, slab type or beam-slab type. Within this study an irregular cantilevered staircase is studied and its structural dynamic characteristics are explored. An eigen-value modal analysis is performed to determine the mode shapes, their corresponding modal participation factors and natural periods of vibration. The highest modes of interest are identified. Following on that, a scaled record of a previous historical earthquake was applied within a direct integration time-history analysis in order to assess the performance of that irregular stair case under earthquake loading. That load was applied within two perpendicular load cases representing the different possible directions of earthquake vibrations in which the effect of the structure irregularity was assessed per direction. The structure was found to have large drifts creating a need to change the structural system.