iRule to redirect based on context

The easier option may be to put each of these internal IP addresses into separate pools and then do something like this:

when HTTP_REQUEST {
    switch -glob [string tolower [HTTP::uri]] {
        "/123*" { pool pool_123 }
        "/456*" { pool pool_456 }
        "/789*" { pool pool_789 }
    }
}

Where "pool_123" is the pool that contains the server with ipaddress1, etc. You're not technically redirecting but rather proxying the traffic. The client will only see the URL that they entered.