sandyford has the edge over blanch - in terms of location at least IMO. Its got good public transport in the luas - that green line is a great service, only 20 mins from town. And its close to the m50 there too.
however i couldnt see many ppl from the n side going too much out of their way to travel accross when they can get most things online. And visa versa for ppl on s side heading too far north.
But can you justify setting up a specialist shop somewhere like sandyford. Will you not be getting people who already use online shops. In order to gain new customers i would have thought you would need to locate somewhere there is a large body of shoppers on a daily basis. Ie town or one of the shopping centres.