Reconfigurable intelligent surface (RIS) is an emerging technology to improve the spectral efficiency of wireless communication systems. However, the increase of RIS elements results in the non-negligible overhead of channel estimation and channel feedback, as well as the high complexity of beam design. Therefore, how to improve the system capacity with a limited number of RIS elements becomes a challenge. In this paper, we propose a brand new irregular RIS structure to enhance the capacity of RIS-aided wireless communications. The key idea is to irregularly configure a given number of RIS elements on an enlarged surface, which provides extra degrees of freedom and spatial diversity compared with the classical regular RIS. For the proposed irregular RIS-aided communication, we then formulate the joint topology and beamforming design problem to maximize the system capacity. Accordingly, we propose a joint optimization framework with low complexity to alternately optimize the RIS topology and the corresponding beamforming design. Finally, simulation results demonstrate that the proposed irregular RIS with a limited number of RIS elements can significantly enhance the system capacity compared with the traditional regular RIS.