Originally Posted by sgm
AFAIK you must clear immigration in LGW as Dublin is considered a domestic flight due to the common travel zone. I could be wrong but if you turn up for the Dublin flight without that barcode you are sent back to immigration. I saw this happen to a person coming from Australia and connecting to DUB via LGW. Seemed to cause alot of drama.
Correct. The UK and Ireland operate a common travel zone (no paasport checks between the two countries), so if you're heading to Ireland via the UK you clear immigration in the UK. You will clear customs at your final destination (DUB in this case).