Some basic facts:
The Common Travel Area (now CTA) includes the United Kingdom, Guernsey, Jersey, the Isle of Man and Ireland.
Between the UK, Guernsey, Jersey and the Isle of Man, the border is fully open without immigration control. In addition, Guernsey, Jersey and the Isle of Man apply the UK entry rules and issue UK entry stamps on passports. As a result, the UK is now required to include these territories.
With Ireland, the situation becomes more complicated as Ireland is do not fully in line with CTA's concept of open borders.
More specifically, non-EU / Schengen citizens must receive an Irish entry stamp, even if they arrive from the UK. For visa-free nationals, this will allow them to stay in Ireland for up to 3 months, while for Irish visa holders visa requirements apply.
On the contrary, those arriving from Ireland to the United Kingdom are do not receive an entry stamp in the United Kingdom. Non-visa third-country nationals who have entered the CTA by Ireland are allowed to remain in the whole of the CTA (including the United Kingdom) during the period allowed for entry into Ireland. British visa holders being subject to the visa requirements in force in the United Kingdom.
Travel Ireland-United Kingdom
If you enter the CTA via Ireland, non-EU / Schengen citizens receive an Irish entry stamp. If they also hold a British visa (or are exempt), they can then continue their trip to the UK without any formalities. Although on-the-spot checks take place at British seaports and at the Irish land border, no British entry stamp is issued and none are required.
On the other hand, this means that the time spent by travelers without a British visa in Ireland is also allowed to stay in Ireland, which also includes the rest of the CTA. For travelers exempt from the visa requirement for the United Kingdom and Ireland, this exemption is 6 months in the UK, but only 3 months in Ireland; as such, if for example A US citizen visitor enters the CTA via Ireland; they only have three months in the whole CTA.
Travel United Kingdom-Ireland
In Irish airports, all international arrivals will pass control of the Irish border. As a result, passengers arriving from the United Kingdom requiring an entry stamp will receive one without difficulty.
By the sea
Arriving by sea from the UK, border checks are often done, but not always. If there is no check, pedestrians and passengers by car should go to INIS (in Dublin) or Garda (in Rosslare) to receive an entry stamp. If you are traveling by bus, they must contact the border control in advance (for Dublin, BMU@Justice.ie; for Rosslare, +353 539 133 204), indicate to them the bus in which you will take and tell them that you need an input stamp. .
By land of Northern Ireland
As also stated on the INIS website, non-EU / Schengen citizens who enter Ireland via the land border must go to the INIS (in Dublin) or local authorities. Garda station (elsewhere) as soon as possible to get an entry stamp.
Do the undersigned not know if all Garda Stations process foreign entries, or only dedicated entries – this is verified with Garda.