The fast ions produced inside a mirror trap by neutral beam injectioncould form periodic short-lived density peaks near a turning point ifthe injection energy is properly modulated in time. Achievableparameters of fast ion bunches thus formed are analysed in thispaper. The theory is illustrated by estimates for the neutron sourcebased on a gas-dynamic trap. The bunching of deuterium and tritiumions can produce periodic short bursts of neutron radiation with intensity 1.5 times higher than the average level. The modulation ofthe neutron flux could extend the field of application for theneutron source. Also, the bunching could serve as a precise plasmadiagnostic in mirror traps.