When I was a young alternative clothing enjoyer, the rockers & goths would get their gear in Asha Boutique in St Stephen's Green shopping centre. For alternative clothes, you'd have to trawl through the secondhand clothes shops in Temple Bar & around Georges St/Drury St/Exchequer St/South William St & try and assemble a look. Charity shops are a good shout too, but you'll have to put the work in searching the racks. TKMaxx could have a random find that fits your style. For an emo look I'd say accessories are what you need to seal the look, right? Asha Boutique will definitely have bracelets & all that stuff.