A Visa-required national (from Lebanon), usually residing in Canada, obtained a 14-days-visit-visa for Ireland.
(Yes, he will go back to Canada. Yes, this is genuine. He'll receive a Canadian Passport next year, so everything else would be pure madness...)
Next we booked the following flight:
Canada - Heathrow - Dublin - Heathrow - Canada
From previous enquiries we knew that a transit-visa is not necessary in the UK, no matter which nationality, if you hold a Canadian residence permit. (Experience confirmed by flying from Canada-Heathrow-Lebanon-Heathrow-Canada without problems)
Suddenly it occurred to me, that the flight from Heathrow to Dublin is treated as "domestic", and was wondering how he can transit in Heathrow in this case, without a UK Visa.
Also the way back seems strange: Not having a UK Visa, entry into Heathrow from Dublin (even if only to check in to the flight to Canada), may effectively be illegal.
The well-hidden answer is on the last page of the relevant site from UKVisas:
The flight from Canada to Heathrow will depart tonight. Any hope this will work out?Transiting to the Republic of Ireland
Passengers must pass through immigration control in order to take a flight to Ireland. Visa nationals (and passengers qualifying for DATV exemption above) may Transit without Visa providing they fulfil the TWOV conditions and are properly documented for entry into Ireland.
DATV nationals transiting to Ireland must obtain a visit visa - not a Visitor in Transit visa which is only for transit to a destination outside the Common Travel Area (Rules HC395 paragraph 47 refers).
All visa nationals wishing to transit the UK but spend longer doing so than the 24 hours permitted under the TWOV concession must obtain a visitor in transit visa for stays up to 48 hours or a visit visa.