Originally Posted by B747-437B

One thing to note though if you do the journey in reverse is that the Republic of Ireland usually grants leave to enter for 90 days while the UK grants it for 6 months.

If you enter the CTA via Ireland and then travel to the UK without being examined by a UK Immigration Officer, you are restricted to 90 days (or whatever period of stay is granted by the Gardai) within the CTA rather than the 6 months that would usually be granted by default if you enter via the UK. If however, you hold a UK visa or entry clearance with a different condition, that would take precedence for the portion of time spent in the UK, but would be considered to have started on the day you entered the CTA.

eg. US citizen enters the CTA in Ireland and is granted 90 days stay. This 90 day stay applies to the full extent of their stay in both the RoI and the UK.

eg. Indian citizen holding a UK C-VISIT visa (180 day stay in the UK) enters the CTA in Ireland and is granted 90 days stay. The 90 day stay limit applies only to their stay in the RoI and they may stay the remainder of the 180 days granted by the visa in the UK. The 180 day clock however starts from the date that they entered Ireland and not on the date they entered the UK.