Background: Dose modulation is a key factor in practical proton therapy. This study investigates the dose modulation methodology of irregular radiation field (IRF)-based proton therapy using forward radiation treatment planning and conformal dose layer stacking (CDLS) methods. Materials and Methods: The geometric configuration of a virtual multi-leaf system was constructed to generate IRFs during Monte Carlo simulations. Two patient geometries—lymphatic metastasis and brain tumors—were configured to investigate the dosimetric feasibility and applications of IRF-based proton therapy in ideal patient anatomies. The investigated tumors were divided into slices perpendicular to proton beam axis. Segments were designed to be conformal to the profiles of these tumor slices. Conformal dose layers were produced by modulating the proton intensities and energies of the predesigned segments. Then, these dose layers were stacked throughout the tumors to obtain sufficient and conformal tumor doses. Results: From the proposed IRF-based proton therapy, tumors with 4-7 cm extents along the depth direction could be treated with fewer than 10 segments. The lymphatic metastasis and brain tumors were sufficiently covered by 95% dose lines, while appropriate distal and proximal dose conformities were achieved. The maximum tumor doses did not exceed 110%. Conclusions: Theoretically, the proposed IRF-based proton therapy using forward planning and CDLS methods is feasible from the viewpoint of dosimetry. This study can serve as a foundation for future investigations of potential proton therapy methods based on fast conformal dose layer stacking using radiation fields with irregular shapes.