Irule working on current connection.

I agree with Bryce's suggestion. Additionally, you could clean up / optimize the iRule by using a datagroup with the defined networks and use matchclass.

https://devcentral.f5.com/wiki/default.aspx/iRules/matchclass

You could create a datagroup named internal_networks_datagroup with a list of subnets/networks and use something like the following:

If pool members are down
if {[active_members [LB::server pool]] < 1} {
    if { [class match [IP::client_addr] equals internal_networks_datagroup] } { 
        If client is internal send to internal pool
        pool "[LB::server pool]_internal"
     } else {
        Send external connections to maintenance page
        HTTP::redirect "http://www.***.com/Maint_Pages/Maintenance.html"  
     }
}