Skippy,
All you need is here.
Common Travel Area (CTA)
I would imagine you are familiar with this too.
Common Travel Area
15. The United Kingdom, the Channel Islands, the Isle of Man and the Republic of Ireland collectively form a common travel area. A person who has been examined for the purpose of immigration control at the point at which he entered the area does not normally require leave to enter any other part of it. However certain persons subject to the Immigration (Control of Entry through the Republic of Ireland) Order 1972 (as amended) who enter the United Kingdom through the Republic of Ireland do require leave to enter. This includes:
(i) those who merely passed through the Republic of Ireland;
(ii) persons requiring visas;
(iii) persons who entered the Republic of Ireland unlawfully;
(iv) persons who are subject to directions given by the Secretary of State for their exclusion from the United Kingdom on the ground that their exclusion is conducive to the public good;
(v) persons who entered the Republic from the United Kingdom and Islands after entering there unlawfully or overstaying their leave.
FR and most pax will only require photo ID.