How can I find the RMS value of a wave in irregular form?
This might sound a bit strange, but, really:
By calculating the RMS. It's the root mean square! So,
$$\text{RMS}=\sqrt{\frac1{T_{\text{observation}}}\int_{0}^{T_{\text{observation}}} V^2(t)\,\mathrm dt}\,,$$
nothing more, nothing less. It's really that simple!