Check out this post and download the Array.mrc file (it handles the hash tables).  Then setup three arrays, one for first name, one for last name, and one for address.
;; Example
;; Usage: $addClient(first name, last name, address)
alias addClient {
  array -a name $1
  array -a lastname $2
  array -a address $3
}
;; Usage: /delClient [Index]
alias delClient {
  array -d $1 name
  array -d $1 lastname
  array -d $1 address
}
;; Usage: /delAllClient
alias delAllClient {
  array -f name
  array -f lastname
  array -f address
}
;; Usage: $getClientAddress(1)
alias getClientAddress {
  return $array(address, $1)
}
Also if you want this information available after restarting mIRC you will also need to 
save on exit and 
restore on start the following hash tables: Array.name, Array.lastname, and Array.address.
Edit: I will be releasing an updated version of Arrays.mrc to  
mircscripts.org and  
my web site within the next week. It will handle saving the arrays for you...
I'll probably have it ready for release today or tomorrow, but no promises here.