can this Irule be optimized?

This iRule looks good. Maybe just add a [string tolower [HTTP::uri]] in your if statemement:

 
 when HTTP_REQUEST { 
     if { [matchclass [string tolower [HTTP::uri]] starts_with $::legacyPaths] } { 
          checked for matchclass == legacyPaths and forward to Legacy_http_pool 
           snat 172.23.0.60 
           HTTP::header insert X-Forwarded-For [IP::remote_addr] this was done to test i am trying to do it using http profile.. 
           used for debub 
           HTTP::redirect http://www.google.com 
           pool Legacy_http_pool 
       } else { 
            when uri matches admin in Literatum check for source ip 
           if { [string tolower [HTTP::uri]] starts_with "/admin" and (not [matchclass [IP::remote_addr] equals $::Admin]) } { 
               path for admin uri but not internal IP 
               log local0. "External IP ([IP::remote_addr]) attempting to access admin path ([HTTP::uri])" 
               HTTP::redirect http://redirect.com/404.html 
           } else { 
                pool Stage_http_pool 
           } 
     } 
 }  
 

X-Forwarded-For header will be insert in your request after it goes through the BIGIP. So if you want to check this header you need to have a look between the BIGIP and the web server