Tesco often have dress up clothes at knock down prices during their sales. I got a Tinkerbell outfit for my 2 year old for 4.50 and an angel dress with wings for 7 euro.
They have Ben 10 and other stuff for boys too; just go to the bargain rails and have a good root aroundand you might find what you’re looking for! Or you could ask at the customer service desks about when their next sale for this kind of clothing will be on and get in there early that day!
Alternatively, try your local ‘Enable Ireland’ or ‘Irish Cancer Society’ or ‘Oxfam’ shops, they are little treasure trovies for things like that.