I spotted some in the past couple of weeks in TK Maxx both in Blanchardstown and in St Stephens green.
Also Aldi and Lidl have done kids wellies a few times over the past couple of years, so might be worth keeping an eye out.
You can get wellies in Dunnes as small as a size 4, but you have to keep looking, and be lucky! Most places that do kids shoes do kids wellies, but there aren't very many in smaller sizes. What size are your childs feet? Wellies don't go by age, but by shoe size.
I found it terribly difficult to get wellies for my 2 year old twins (size 7) this year and ended up ordering them from the Next online catalogue. Got them delivered free to their shop within a few days. Had to get two pairs the same to avoid rows!
If you are near the North or England ASDA do a great range. I got a pair recently for my 2.5 year old nephew for only 6E, and they did quite small sizes.