An irregular variable is a variable star with no detectable period in its brightness variations. There are two, very different main types, irregular eruptive variables and irregular pulsating variables, neither of which is particularly well understood.
Irregular eruptive variables are divided into three categories. Group I variables are split into subgroups IA (spectral types O to A) and IB (spectral types F through M). So-called IN (irregular nebulous) variables, indigenous to star-forming regions, may vary by several magnitudes with rapid changes of up to 1 magnitude in 1 to 10 days, are similarly divided by spectral type into subgroups INA and INB, but with the addition of another subgroup, INT, for T Tauri stars, or INT(YY) for YY Orionis stars. The third category of eruptive irregulars are the IS stars, which show rapid variations of 0.5 to 1 magnitude in a few hours or days; again, these come in subgroups ISA and ISB.
Irregular pulsating giants or supergiants, all of late spectral types (K, M, C, or S), are classed as type L-LB for giants and LC for supergiants. How many of these are actually semi-regular variables that simply need more study, remains unclear.