Like Craig, I, too, am surprised that you have not been able to find the dry milk powder. In the U.S., the most popular brand of that product is Carnation.
I'd also like to mention that while there have been several members who have reported good results with the recipe you plan to use, there have also been several complaints that the amount of dough made using that recipe is far in excess of what PH used in its stores. Some time ago, I took a stab at converting the recipe you plan to use to the amount, 22 ounces, that PH apparently used for its 14" pan pizzas. You can see my converted dough formulation at Reply 6 at http://www.pizzamaking.com/forum/index.php/topic,4607.msg38909.html#msg38909
You might also take a look at the thread at http://www.pizzamaking.com/forum/index.php/topic,12806.0.html
. In that thread, at Reply 15 at http://www.pizzamaking.com/forum/index.php/topic,12806.msg132091.html#msg132091
, I came up with a dough formulation that was essentially a cross between the PH pan pizza and one made by Godfather's, another popular pizza chain that makes a pan pizza. That recipe also calls for dry milk powder but if you look at Reply 17 in that thread, you will note that member Jet_deck asked me to substitute evaporated milk for the dry milk powder, which I did. You can see his results starting at Reply 25 at http://www.pizzamaking.com/forum/index.php/topic,12806.msg132648.html#msg132648
. So, if you have access to evaporated milk where you live in Ireland, the PH/Godfather's recipe may be another viable option.