Looking at the offers on the aer lingus website, it seems cheaper to book two one way flights, taking advantage of their US$ offers (presumably aimed at Americans). For example, Boston to Dublin on 28 April is $276 (€175). If you book in Euro you will pay €235. The savings are even bigger on other routes / dates.
Have they some way of restricting the US$ offers to US residents only?
As far as I know there is nothing to stop you doing it. I booked a one way flight back from the States before with them using my Irish credit card and Irish address. Might have changed since
I did it for my mam and her friends last December. I saved them all €50 each by doing it. I was worried immigration might comment on the one way ticket - possibly single them out for further questions etc but nothing happened. They flew out and back without incident.
You can't - fares are calculated on the point of origin.
Remember, these discrepancies are not to do with the weak dollar, for the most part - they are to do with the relative strengths of the markets where the travel originates.
Even if you called a US travel agent to get a return DUB-USA fare, while you would be charged in dollars, you would be paying the higher exDUB fare, pluse commissions etc.
If you are flying to the US with Aerlingus you are always better of buying the outbound and inbound leg separately. Your ticket from US to Ireland will be cheaper, will be priced in euros ( and therefore even cheaper) and there are no restrictions, in terms of address or anything. You will not have to pay "handling fees".
When you are about to confirm your purchase you will be asked to accept a price in euros (basically a conversion done by Aerlingus based on their conversion rate) or pay in dollars and have you credit card company do the conversion. My experience is that the latter is the better option. My flights were $760, Aerlingus conversion was € 514, I just saw my statement and paid € 492... That is € 22 earned in a split second.