iRule checking Pool is down

I don't understand what you propose very well, but it might help you:

when HTTP_REQUEST {
    set up 0
    set down 0
    set disabled 0
    foreach member [members -list [LB::server pool]] {
        set status [LB::status pool [LB::server pool] member [lindex $member 0] [lindex $member 1]]
        switch $status {
            "up" {
                incr up 1
            }
            "down" {
                incr down 1
            }
            "session_disabled" {
                incr disabled 1
            }
        }
    }
    log local0. "===> [LB::server pool] members (Active: $up) (Down: $down) (Disabled: $disabled)"
    unset up down disabled
}

Regards.