score:2
Assuming you want to see some of the city's highlights, but don't want to walk too much, then I'd say your best bet is a hop-on, hop-off bus tour. This will let you see much of the city, learn about it (there's a guide on board giving commentary), and will let you get between the tourist destinations you might want to visit.
I've heard good things about the Hop on, Hop off bus run by Dublin Buses, who have two routes covering the main sights, with one ticket between them. They also run the quicker of the two buses from the airport to the city, and have friendly staff at the airport who can sell you a combined ticket for the airport bus and the hop-on hop-off bus. At the airport, you want the green double-decker Airlink bus, not the Blue airport express bus. The airport bus driver can advise you where to get off in the city centre to change to the tour bus. Journey time is about 30 minutes from the airport to the city, and there's 3-4 buses an hour, so you should have a decent amount of time in the city centre.
Depending on weather and jetlag and energy levels, you can select where to get off at, then carry on round to the next place of interest. Trinity College is well worth a visit, which the bus goes to, and I'd say the National Museum of Ireland (free!) is a must for anyone interested in Irish History from about 500 to 5000 years ago. Beyond that, there's loads else to do, but your specific interests will govern which to do.
If the weather was good and you were all a bit younger, I'd suggest taking the airport bus then doing an organised or self-guided walking tour, probably including a museum/gallery or two. With lots of walking being a potential issue, paying a bit extra for the bus seems worth it.
Upvote:2
There is no DART that serves the airport (at least there wasn't 2 years ago). You'll have to take a bus or a taxi to get to Dublin center.
As to where to take your oldies, it is kind of hard to suggest without knowing them or what sort of activities they like to do. Usually old people tend to be more religious, so perhaps, showing them the St Patricks Cathedral (and St Patricks Park on the side) might be a good idea. Dublin Castle is not far from the Cathedral (within walking range if they do not have any health condition that impairs them from walking) and is also a nice place to see. Lots of history and nice old buildings.