This reply to Sylvia Hermon’s question seems to raise more questions than it answers, but the Home Office minister’s reply gives the impression at least that if you have Irish nationality you are not obliged to carry an ID Card in the way that, say, a French citizen would be obliged to after three month’s residency in the UK.
Lady Hermon: To ask the Secretary of State for the Home Department whether Irish nationals living in Northern Ireland will be required to carry a UK identity national card once they are introduced. 
Joan Ryan: Under Section 13(3) of the Identity Cards Act 2006, there is no authorisation to make any regulations that would require anyone to carry an identity card with them at all times once they are introduced.
The Belfast (Good Friday Agreement) confirmed that all the peoples of Northern Ireland had the right to identify themselves as Irish, British or both, as they chose.
As such, those Irish nationals who wish to identify as a dual national may choose to obtain a British passport and, in that event, would be issued with an identity card as well. The status and entitlements of those who choose not to obtain a British passport, whether they identify as Irish, British or both will remain unchanged.
However, it is the Government’s policy that, eventually, any individual who is resident in the United Kingdom for more than three months should be issued with an identity card, regardless of their nationality. Further primary legislation will need to be approved in Parliament in order to introduce such a compulsory scheme.