OP you seem to be making a huge meal out of an incredibly simple process... Go onto skyscanner.com or kayak.com, plug in the dates you are interested (both also allow you to search round that date for cheaper deals) and book the first reasonable option... Currently the cheapest flight from Dublin for the end of September-October is one changing at heathrow, with BA on the way out and Air Canada on the way back. That's £430.
All airlines are much of a muchness in economy (in the last year I've flown Swiss, Lufthansa, BA (several times), Qatar and a few small regional airlines).Air Canada are fine too. There may be slight variations in IFE but not enough to really pay more. BA are a good airline, no worse than Qatar IME (who are a 5* airline). If you want to sit with a travel companion and somehow can't do that when you check in the night before just ask at the desk and they can usually swap your seats so you can sit next to the people you are travelling with. No need to pay to sit places!
If you want to fly premium you can look for prices on the websites as well. I guess it's worth going to a travel agent if you want but with the proliferation of online search tools (ita matrix is another one) there really is no need. They usually end up either more expensive or at best the same price... I gave up on them a while ago.
Edit: meant for Btone not the OP, I thought he was the OP...