This paper presents the optimization and evaluation of parallel I/O for the BIPS3D parallel irregular application, a 3-dimensional simulation of BJT and HBT bipolar devices. The parallel version of BIPS3D employs Metis, a library for partitioning graphs, finite element meshes, or sparse matrices. First, we show how the partitioning information provided by Metis can be used in order to improve the performance of parallel I/O. Second, we propose a novel technique, called Interval Data Grouping (IDG), which exploits the data replication of mesh nodes for optimizing the scheduling of the parallel file operations. Finally, we evaluate the parallel I/O version of BIPS3D for various existing parallel I/O techniques and present an in-depth analysis of the IDG performance.