Trevor Smith, a Lib Dem peer from York has a piece in the excellent Our Kingdom (a must read for any unionists who still care about the integrity of the link between the constituent parts of the UK) on the eborders issue. In particular he touches on the dilemma for both parts of Ireland of where the border might be drawn:
If the e-borders are drawn around the UK to include Northern Ireland, that will be welcomed by political Unionism, if less so by Northern Irish business. Equally, if they are drawn round Great Britain, excluding Northern Ireland, that would be more acceptable to Irish nationalist sentiment but risks alienating the Unionists; a classic illustration of the zero-sum nature of politics in Northern Ireland.
Contriving a system that would cover both the UK and all -Ireland is not without its difficulties: while it would preserve the CTA, it would strengthen the link between the Republic and the UK and would thus detract from the former’s more recently acquired identity as an enthusiastic member-state of the EU – and that would create a political problem for Dublin. Indeed why should the Republic accept this if it seems like a step backwards?