LONDON (Reuters) - Ireland is trying to force the United Kingdom, or at least Northern Ireland, to remain in the customs union after Brexit as it suits their national interest, the deputy leader of the province s Democratic Unionist Party said on Friday.   Their real aim is to try to get to a situation where either they try to force the United Kingdom as a whole to stay within the customs union, which is in their interests clearly,  Nigel Dodds told Sky News.   Or, if they fail that, to at least force Northern Ireland to stay within the customs union and the single market, follow the rules of it, something then we d have no say over, but we d have to abide by the rules which would then bring about a united Ireland much easier.   