possible to have multi-layered irules ?

yes, you could do something like that by using variables to do so. I would be careful at the maintainability of such a solution. But basically:

Rule 1

priority 100
when RULE_INIT {
  set static::trigger 0
}
when HTTP_REQUEST {
  switch [LB::server pool] {
    "pool1" { set static::trigger 1 }
    "pool2" { set static::trigger 2 }
    "pool3" { set static::trigger 3 }
  }
}

Pool1 Rule

priority 110
when HTTP_REQUEST {
  if { $static::trigger == 1 } {
    do stuff
  }
}

Edit->actually, LB::server pool probably won't be very accurate in HTTP_REQUEST, so you might need to move some of the logic to a later event. The flow from iRule->iRule mapping stands though.