The master/servant relationship is a key one when it comes to comic plots in Shakespeare's play.  It is kind of hard for us to grasp this since the idea of masters and servants doesn't fit so much in our modern society as a literal concept.

The irony, if there is any, is that Grumio seems so cowering and obedient in earlier scenes with Petruchio, but bossy and even aggressive with the other servants.  The reason I'm not convinced that this "change of character" is ironic, is that:

1. The audience can't be sure that the behaviour displayed by Grumio earlier isn't just an act for Petruchio's benefit; and
2. It would have been very expected and not ironic at all for there to be a servant pecking order, which would have made it very natural for Grumio to be sort of the "head servant" that bosses the other servants around.

This "split personality" is showcased for comic effect in Act IV, Scene i.  Grumio beats up on Curtis, orders everyone else around, and then scurries to hide when Petruchio enters.  This is meant, I think, more to be good old fashioned comedy, rather than irony.