Need to write Irule to decode auth string to Base64 and fetch domain name from it and compare that domain name with certificate to deny/permit traffic

Right, so minor modification required:

when HTTP_REQUEST {
    if { [HTTP::header exists Authorization] } {
        set creds [HTTP::username]

        if { $creds ne "" } {
         credentials will ether be in domain\user:password or user@domain:password format
            if { $creds contains "\\" } {
                set domain [findstr $creds "" 0 "\\"]
            } elseif { $creds contains "@" } {
                set domain [findstr $creds "@" 1]
            }

            log local0. $domain
        }
    }
}