Possibly they only have 1 left at the 870 price. Airlines sell seats on flights at different prices. Eg. The first 10 seats sold may cost X, while the next 10 may be 2X, the next ten being 3X and so on......
Even the price for one person sounds crazy, we fly with Delta through Atlantas a lot and wouldn't expect to pay more than €420 return for Dub-Atl-Chicago or New York.
We almost always find that we get a better price and/or connections from Delta on the phone than online. Try and time your call so you get the US call centre rather than the Indian one, it makes a difference!
One other thing that strikes me is that you may be pricing the flights too soon? Even when we've gone last minute (for a funeral) Dublin-Atlanta-Chicago cost under €600 pp and that was with 3 days notice, in March last year.
I always thought that the earlier you book the better chance you had of securing a cheap seat - given that we will be travelling around Easter '08, I don't think the flights are going to get any cheaper.
Have tried ebookers and flights with Delta are cheaper there than on the Delta website.
Club Travel have some good prices although not always on a direct flight. Will have to take a closer look
Definitely expenisve; I am flying to Las Vegas (Dub-JFK-LAS-ATL-DUB) later in the year and it cost me €590 including taxes. However, this was after twiddling around the dates and routes. For example, if I flew via JFK on the way home on the same day, it would have cost me €780 rather than the above when flying home via Atlanta.
Sometimes the cheapest price may route you through Paris CDG which is a bit out of the way but it could save you a lot of money if you don't mind the hassle.
If you are flexible with your dates and don't mind where you connect, you should get a decent price.