The withdrawal agreement between the UK and the European Union recognises the common travel space in its protocol for Ireland and Northern Ireland. Article 2 provides for the sustainability of the common travel area. A professor of migration law has said that the continuation of the rules on the common travel area seems compatible with EU law. It has been proposed that Brexit should be an opportunity for a comprehensive agreement on the common travel area. The agreement was provided for by BRITISH law by designating the Irish Free State as part of the United Kingdom for the purposes of immigration legislation.  It was fully implemented in 1925, when legislation in both countries provided for recognition of the landing conditions of other countries for foreigners.  This can be considered the culmination of the CTA – although it was not so called at the time – since it was almost in a common immigration zone. A foreigner who had been admitted to one state could, unless his admission was conditional on not entering the other state, travel to the other state with minimal bureaucratic requirements. In 1985, five Member States of the European Economic Community at the time signed the Schengen Agreement on the phasing out of border controls between them. This treaty and the 1990 Convention on implementation paved the way for the creation of the Schengen area, which was implemented in 1995, and until 1997, all EU Member States, with the exception of the United Kingdom and Ireland, had signed the agreement. The Treaty of Amsterdam, which was drafted this year, transposed Schengen into EU law and gave Ireland and the UK an opt-out allowing them to maintain systematic passport and immigration checks at their borders. The text of the treaty subordinates Ireland`s opt-out to the removal of border controls to the maintenance of the common travel space. One of the main reasons for the Schengen Agreement was to facilitate the daily cross-border movement of workers within the framework of the free movement of workers.
The United Kingdom and Ireland have a short land border, which belongs only to part of the United Kingdom, which makes this reason less important to them. The Common Travel Area (CTA); The Irish territory of Comhlimistéar Taistil is an open border territory that includes the United Kingdom, Ireland, the Isle of Man and the Channel Islands. British overseas territories are not included. On the basis of agreements that are not legally binding, the internal borders of the CTA, if any, are subject to minimal controls and can normally be overtaken by British and Irish citizens whose identity documents are minimal, with some exceptions.   The continuation of the CTA implies cooperation between the British and Irish immigration authorities. The British border force does not carry out routine checks on the immigration of travellers (regardless of nationality) arriving in the UK from another part of the CTA.  However, since the Channel Islands are VAT-free, the British border force carries out selective customs checks on travellers arriving from there. Irish and British citizens have the right to live, travel, work and study in the common travel area. Irish and British citizens have the right to live, travel, work and study under the CTA. The rights of Irish citizens have been recognised by the UK Immigration and Social Security Act (EU Exit Act 2020).
2.1 Between 1801 and 1922, Ireland was part of the United Kingdom of Great Britain and Ireland. Despite the creation of the Irish state in 1922, the Irish were still regarded as British subjects from the point of view of the United Kingdom. As such, they had the same rights as British citizens in the UNITED Kingdom. Historically, after Ireland`s independence, the Irish continue to have a special status in the United Kingdom, which has resulted in many of the CTA agreements/rights being implicit and stem from that status and not from specific legislation or agreement.