I think the issue, put simply, is that the peace in Northern Ireland is extremely delicate and it wouldnt take much to set the troubles off again. Part of the settlement is the illusion of a united Ireland provided by the fact there is no border. It isnt a united Ireland, but anyone can travel between the north and the south at will, trade can occur between the two sides without impediment. The symbolism of a border in Ireland would be bad enough, but it would have practical implications as well. As I said its a very delicate and complex situation and there are better places to research this question than on here. Needless to say, whatever your or my level of understanding of it is, it IS a huge issue, EU Ministers say it will be the single hardest problem to resolve.