  We explore various instability models for the onset of star formation in
irregular galaxies. Critical gas densities are calculated for gravitational
instabilities with a thin, pure-gas disk, and with a thick disk composed of gas
and a star-like fluid. We also calculated the stability properties of three
dimensional systems including dark matter, considered the thermal state of the
gas, and used a modified threshold column density written in terms of the local
rate of shear instead of the epicyclic frequency. The model predictions were
compared to the azimuthally-averaged present day star formation activity traced
by the Halpha surface brightness, and to the 1 Gyr-integrated star formation
activity represented by the stellar surface brightness.
  The ratio of the observed gas density to the critical gas density is lower by
a factor of 2 in most of the Im galaxies than it is in spiral galaxies. Star
formation ends before this ratio drops significantly in the outer regions, and
it remains high in the inner regions where the ratio is often low. These
results suggest that the critical gas density does not trace star formation
with the same detail in irregular galaxies as it appears to trace it in giant
spiral galaxies. The only azimuthally-averaged quantity that correlates with
the current star formation activity in irregulars is the stellar surface
density. Theoretical implications are discussed.
