We present the first investigation of how the cortical regions of the brain respond to the sensations related to oral irritation, using functional near-infrared spectroscopy (fNIRS). fNIRS is used to record irritant-induced activities in the prefrontal and somatosensory regions of nine healthy individuals. Two types of solutions, irritant-free and soft tissue-irritant-contained, are adopted as the stimuli for the control and task experiments, respectively. Our findings reveal that both somatosensory and prefrontal regions show activity as a result of oral irritation. Furthermore, using moving window analysis, we identify the time interval during which the largest number of channels (indicative of high involvement of cortical regions) show irritant-induced activity. Our results indicate that fNIRS can be used to study brain activities related to oral irritation.