  We compute the tree-level bispectrum of 21cm intensity mapping after
reionisation. We work in the directly observable angular and redshift space,
focusing on equal-redshift correlations and thin redshift bins, for which the
lensing contribution is negligible. We demonstrate the importance of the
contributions from redshift-space distortions which typically dominate the
result. Taking into account the effects of telescope beams and foreground
cleaning, we estimate the signal to noise and show that the bispectrum is
detectable by both SKA in single-dish mode and HIRAX in interferometer mode,
especially at the lower redshifts in their respective ranges.
