The train can get you downtown fairly quickly - 15-20 min I think. But I wouldn't risk it or add the hassle myself. There is very little around O'Hare airport. However, the airport itself is worth exploring. Each of the different terminals has different shops, services, even a small church!
I'd also suggest you purchase a day pass to your airline's FF club, if you don't already have access. The Admirals Club for instance (AA) at O'Hare is huge. It has a leather seating, a bar, limited menu restaurant, computers to use, free newspapers, tv's etc. It will make the extra hours go by quickly.You also could get in if by chance you are flying business or first class.
Good advice from Mark & Colleen. 4-5 hours sounds like a lot but when dealing with airports, security, lines, etc. it may be best to stick to the airport. By the time you get somewhere it will be time to return again.
There is nothing close of interest, just the maze of the suburbs. That is the airport I fly out of, and I would also caution you not to even think about trying to leave that area with only four or five hours to spare. If you are not certain where you are, there is a good chance you might get turned around on a train and then be in a rush getting back to the airport. Chicago is a great place to see -- just not in just a few hours. Buy a local paper, have a beverage, and relax. You have a long flight to Ireland ahead of you!
Another vote not to leave the airport. Expect the train to the city to take a 1/2 hour without any distractions.
I often stop by the lounge in the Hilton that is attached to the airport. It's much quieter than rest of the airport. The bar there has a sports theme, so it's casual. There are some nicer shops and a gym (quite expensive for a day pass if I remember correctly) near the Hilton as well.
About 1.5 to 2 hours before the flight, I catch the monorail to the International terminal (a 5 minute ride). Then it's a peek in duty free and through security for some time hanging out before the flight.
Each terminal has different art on display, so it can be nice to walk around the terminals reading the plaques and whatnot.
Another O'Hare frequenter here, and I agree--don't leave the airport!
My home airport is a teeny one, which results in frequent long layovers at O'Hare. (It's either that or a ridiculously short one that leaves me sprinting through the airport to catch my connection).
If I don't have much carry-on luggage with me, my favorite way to pass my O'Hare time is by getting in a good long walk. Two weeks ago I did a good paced walk for about an hour and a half. A good walk would also help me sleep on the long flight.
Tell everyone at Murphy's Hello for me and have a dram of whiskey for me, as well! I hope that your trip is all you hope for and one hundred times more! I look forward to your trip report upon your return!