This is precisely why it makes immigration issues complex. If you are an EU national or someone with the right of abode/permanent residence in the UK (indefinite leave to remain), then you have no problems. If, however, your immigration status requires you to be admitted into the country (given leave to enter the UK), the problem is that there is no-one to actually give you that leave as there are no UK immigration officers either at DUB or at the notional border points between the South and Northern Ireland, so you could find yourself an unintentional illegal immigrant in the UK. Not sure whether there are any possibilities for regularisation currently or if you are required to enter via a UK immigration point in such a situation.