If you enter the United Kingdom through Ireland, you will pass through passport control at your port of entry into Ireland, but you are not required to clear UK passport control. However, you will only be limited to a stay of three months in the UK and Ireland (or whatever the passport control officer in Ireland gives you a leave to remain for) if you qualify for a visa exemption, notthe usual six-month stay in the UK for visa-exempt nationals. Hence, especially if you attempt to enter the UK as a Student Visitor (i.e. a visitor studying for up to 6 months), you should not transit through Ireland unless you possess a valid UK visa or entry clearance permitting a stay of more than three months or intend to stay in the UK for fewer than three months.
If you require a visa for either Ireland or the UK, however, you must possess a visafrom each country that requires you to have one if you intend to visit both of them. Not passing through passport control does not exempt one from having a visa if needed, and you can be fined and deported for not having a visa if discovered.
In addition, no passport control checks are in place from the Channel Islands and the Isle of Man to the UK.
The United Kingdom is physically linked to two other countries. The Channel Tunnel connects the UK to France and Northern Ireland shares a land border with the Republic of Ireland.
While the UK is a member of the European Union, it does not fully implement theSchengen Agreement, which means that travel to and from other EU countries (except Ireland) involves systematic passport / identity card checks at the border and separate visa requirements for several countries.
Almost all passengers travelling to the UK from outside Ireland, the Channel Islands and the Isle of Man go through systematic passport/identity card and selective customs checks carried out by the UK Border Agency on arrival in the UK. However, those travelling by Eurostar from Paris Gare du Nord, Lille Europe, Calais-Fréthun and Brussels Zuid-Midi stations and by ferry from Calais and Dunkirk undergo UK passport/identity card checks in France/Belgium before embarkation and selective customs checks on arrival in the UK. Those entering the UK by Eurotunnel from France go through both UK passport/identity card and UK customs checks in Coquelles before boarding the train.