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(1) Background: To compare the degree of conversion of resin cores in various types of cavities and determine an effective irradiation method for achieving a higher degree of conversion. (2) Methods: Four different-sized cavities (narrow–shallow, narrow–deep, wide–shallow, and wide–deep) were simulated using a Teflon mold. The light irradiance reaching the bottom of each mold was measured by positioning a radiometer. The degree of conversion of the dual-cure resin core after irradiation (400 mW/cm² for 20 s, 400 mW/cm² for 40 s, and 800 mW/cm² for 20 s) was measured using Fourier-transform near-infrared spectroscopy. (3) Results: The highest light irradiance was found at the bottom of wide–shallow cavities, followed by wide–deep, narrow–shallow, and narrow–deep ones (<i>p</i> < 0.001). In narrow cavities, irradiation at 800 mW/cm² for 20 s led to a significantly higher degree of conversion (<i>p</i> < 0.001). In wide cavities, irradiation at 400 mW/cm² for 40 s and 800 mW/cm² for 20 s both led to a significantly higher degree of conversion (<i>p</i> < 0.001). (4) Conclusions: Less curing light reaches the bottom of cavities with a smaller diameter and greater depth. Providing a higher irradiance of light can induce a higher degree of conversion of resin composites in narrower cavities.