In Dublin, I think it's considered, if not rude, at least not the height of courtesy, to use an umbrella in dense areas where you will be bumping in to people and essentially pushing your water on to them.

However, my feeling is that the real lunacy here is having very narrow pavements (sidewalks) carrying dozens or hundreds of people while having streets, which might be carrying ten or twenty people along the same stretch of road in taxis (which most cars are in Dublin city centre on a weekday morning), get 8-10x as much space. (I am much more sympathetic to buses).