We present a perfectly matched layer (PML) absorbing boundary condition that can be imposed along an arbitrary geometrical boundary in 3D elastic wave modeling. The scheme is developed by using the local coordinate system-based PML splitting equations and integral approach of the PML equations under a discretization of tetrahedral grids. However, no explicit coordinate transformations arise. The local coordinate system-based PML splitting equations make it possible to decay incident waves around the direction normal to the irregular geometrical boundaries, instead of a coordinate axis direction. Based on the resulting 3D irregular PML model, we can flexibly construct the computational domain with smaller nodes by cutting uninterested zones. This results in significant reductions in computational cost and memory requirements for 3D simulations. By building a smooth artificial boundary, the irregular PML model can avoid the respective treatments to the edges and corners of the artificial boundaries. Also, the irregular PML model may reduce the grazing incidence that makes the PML model less efficient by changing the shapes of the artificial boundaries. The numerical examples demonstrate performance of the irregular PML model.