My understanding used to be in line with what the wikipedia article says:
But then I saw a small paragraph in a BBC news report about a Brazilian who was killed in the UK after entered from Ireland:Other nationalities
While the Common Travel Area has, for most of its history, involved an open or relatively open border, this has never meant that someone who legally entered one part of the area was automatically legally entitled to enter another part. The Area, unlike the Schengen Agreement, provides no mechanism for a common visa system. The United Kingdom and Ireland operate entirely separate visa systems with distinct, though similar, entry requirements.[citation needed] A United Kingdom visa or work permit will not allow a traveller entry to Ireland, nor vice-versa. The Channel Islands and the Isle of Man allow entry to holders of United Kingdom visas (with some exceptions).
A recent UKBA FOI answer confirms the previous quote:The court heard how as a person entering Britain from Ireland, he would have had an automatic three-month leave to remain which at the earliest would have run out on 23 July, the day after he was killed.
Interestingly UKBA page on the CTA is very general but says something different:UKBA FOI answer wrote:Travelling from Ireland would make matters slightly more complicated in that there is no routine immigration control between there and the UK, due to what is called the common travel area (CTA). The CTA operates on the principle that once a person has been granted leave to enter in one part of it, they will not normally require leave to enter another part of it whilst that leave is still valid and provided that they do not leave it. Consequently, so long as you are [not] subject to one of the exceptions to this general principle (excluded, previously deported etc.), you would not need to [do] anything further in order to travel [to the UK] from [Ireland] for a visit.
Does anyone understand how exactly this works? Does it only apply to (e.g.) Brazilians (who do not require UK visa), or does it also apply to a Russian citizen who has entered Ireland legally and who does not have a separate UK visa?However, there are exceptions to this principle - for example, a visa national will need to have a visa if they enter the UK from another part of the CTA. You can find more information about visa nationals under 'More information' below.