In terms of the uprising, I think that the leaders of the IRB may have managed to get them smuggled in from another country. Either that or the were able to steal them from a near by barracks. Bearing in mind that Michael Collins effectively used this tactic all the way through the independence war to acquire weapons, I believe that there would have been continuity of methods. Also when you look at the GPO, the building was very big and had a gated courtyard. Effectively this would be allowed people in the uprising to position themselves to look armed. What I imply by this is that the men with weapons would more than likely be in view to the British, whilst the others would be further out of sight so that nobody could see they were not armed. Alternatively, Aemon De Valera's partial American citizenship may have helped, as he probably could've gotten them shipped in and make it look legitimate