Carbon dioxide concentrations in Ballynamintra Cave, S. Ireland, generally increase with distance from the entrance, but this trend is non-linear because physical constrictions and slope changes compartmentalize the cave into zones with distinct PCO2 signatures. In this cave, CO2 originates from the soil and enters the cave by degassing from dripwater and by seeping through fractures, and is then transported throughout the cave by advection. Elevated concentrations in roof fissures, joints, and adjacent to walls suggest that these locations shelter CO2 gas from advection and permit local accumulation. CO2 enrichment was noted over a sediment accumulation, suggesting that microbial oxidation of organic compounds in the sediment provided an additional CO2 source distinct from the soil zone above the cave. Advection driven by external barometric pressure variations caused ventilation, which is the principal CO2 sink. The data presented here underscore the need for high resolution data to adequately characterize cave air PCO2 variability.