There is a dance shop on Talbot street, towards the connolly station end. From the window display which I pass every day, it seems to supply all sizes and types of dance shoes
Both the shop in Ranelagh and Parnell st are branxhes of Dance world. Obviously for a small child, you need to go to the shop to get shoes fitted, but for toher stuff, their website www.danceworld.ie is useful.
I think there is a bit of a scam with these shops, whereby some ballet schools will only accept uniforms from these shops, instead of the much cheaper uniforms available in M&S.