Most chain stores and department stores stock dresses up to size 16, including Penny's, Dunnes etc. Some Penny's stores stock up to 20 (I think) in their "Prima Plus" range. There are also a few specialist stores such as Evans and LiL in Dublin that stock larger sizes again.
The recent British multiples that have arrived in Dublin (Next, Principles, Dorothy Perkins Debenhams and of course Marks and Spensers) are also good places to look. Debenhams and M&S in particular are meant to have good names for trans customers.
Shoes are always a slight problem for the less than svelte, Next do some styles upto size 9 and some of the larger manufacturers (Ecco, Clarkes) do go as far as 9 in some (but not all) styles. Pennys do up to size 8 in all styles, but Dunnes tend to stop at 7 or 7.5. Evans do carry shoes in some of their shops (Henry Street and the Liffey Valley store for instance) and they go up to size 10 in all styles, and some styles are available in size 11.
I've since discovered that some of the Barratt chain of shoe-shops carry a few styles up to size 10, but not all shops and not all styles.
Zerep on Grafton Street have a small range of large sized shoes, again going up to a size 10 while Cleary’s shoe department sometimes carries shoes up to a size 12 but not often enough to be relied on.