If you are challenged at a CTA port (which is unlikely), they will scan it and if nothing flags up, you can proceed. You might get some stick about upgrading to the newer ID’s, but that’s reasonable. Keep your DVLA reference number handy.
If you are an ROI citizen you are covered on UK side by the Ireland Act 1949 http://cain.ulst.ac.uk/hmso/irelandact1949org.pdf On the ROI side there is a treaty agreeing to the Act. These are more of an academic curiosity because the provisions are so widely known anyway. The ROI Act is discussed here http://en.wikipedia.org/wiki/Republic_of_Ireland_Act_1948
For re-entry into the UK, you also benefit from Paragraph 18 of the UK immigration rules, and they are SUPPOSED to assure that P18 still applies to you, but it’s very doubtful that things would reach that point.
Credit:stackoverflow.com‘