can we direct traffic based on IP address

You might be better off using two separate Virtual Servers and the Priority Group Activation feature but if you really must have an iRule;

when CLIENT_ACCEPTED {
 If the source IP address is in the internal network;
 if { [IP::addr [IP::remote_addr] equals x.x.x.x/xx] } {
    continue and check if both Pool members are up
    if { [active_members pool_name] > 1 } {
        if so, select our prefered pool member for internal traffic;
        pool pool_name member x.x.x.x
    }
    else {
        if not, exit and let the standard load balancing method do it's thing
        return
    }
 If not an internal, assume it's an external IP and check if both Pool members are up
 elseif { [active_members pool_name] > 1 } {
        If so, select our prefered pool member for external traffic;
        pool pool_name member x.x.x.x
    }
    else {
        if not, exit and let the standard load balancing method do it's thing
        return
    }
 }
}