Two of us travelling from USA to the Republic of Ireland via LHR. One of holds a USA passport and one of us holds a USA and Irish passport.

I understand the Irish citizen must (should?) present himself as an Irish Citizen when entering Ireland. And also should present himself as an EU member citizen when entering and exiting the EU. What I can't quite figure out is when does one enter the EU.

According to the step by step instructions on the Heathrow web site, one goes through passport control at T5 after taking the flight connections bus. Is this an entry into the UK/EU? Have you then cleared immigration such that you could simply walk out of the airport (assuming only hand baggage)? Is this where (if you need one) you obtain your EU visa? Or do you receive that visa when you land in your onward destination? It doesn't seem like the latter, since many on a flight from LHR are presumably UK citizens and don't even have to show a passport in Ireland. Presumably, if the former, the Irish citizen should present the EU passport at T5 passport control on the connection.

What about connections in LHR for onward flights to other EU member countries, other than within the UK or the RoI. Suppose you are connecting to France. Would the USA citizen, after going through passport control at T5, have entered the EU and does he get an EU visa stamp here, or does this happen when he lands at CDG? How about the Irish Citizen -- should he be showing his USA passport or Irish passport to T5 passport control in this circumstance?

Edit: I guess I probably should not be referring to "visas" for the USA citizen, since I guess they don't need them in the EU, but I suppose instead I'm talking about entry and exit stamps.