This question already has an answer here:
I'm going with an American friend from Belfast to Dublin by bus. Despite the lack of border control at the land border due to the Common Travel Area agreement, non-EU/EFTA nationals are supposed to get processed and stamped into Ireland even when entering from the UK - the Immigration section of the Gardai (Irish Police) has consistently told me this by E-Mail and on the phone
I e-mailed the Gardai asking about entering by bus from Belfast, and they said we should go to the Garda Station in Dundalk for entry processing.
However, Belfast-Dublin buses do not stop in Dundalk, but run non-stop between Newry (UK) and Dublin.
What is the proper procedure in this case?
UPDATE: just found the following on the INIS website:
If you arrive via the border with Northern Ireland, you must report to an immigration officer at Burgh Quay Registration Office Dublin or your local Garda Síochána (police) station as soon as possible.
I've noticed a common belief here on Travelstack that, due to the Common Travel Area, you need to do nothing as long as you were admitted to the UK. That is then clearly not the case.