Any ideas for a 6 hr 30 minute layover in Dublin? Is that even enough time to bother leaving the airport?

Assuming you're on time (and immigration is quick), it's certainly enough time to go into the city and do one of the big sites - Guinness Storehouse, Kilmainham Gaol, etc.

For something less full on, Malahide is apparently nice, though I've never been.

Or you could just find an airport bar, have a meal, read a book, etc.! Depends on your energy levels I guess.

Tell us more: What time of day is the layover? Have you been to Dublin before? Ditto for others you're with? Or do you think you'll return in the foreseeable future? If neither, you might consider riding around on one of the hop on - hop off tourist buses and then walking around the central area, perhaps stopping in at a pub for a Guinness for tradition's sake. But it really depends on the answers.

Arrive Dub 9:10, depart for fco 3:45. about 6.5 hours

That's enough time for a mooch about and a nice lunch. There's a couple of coach options that get into the city quite quickly for a reasonable price. I used the Dublin Express on my last visit, April last year.

I'm a big fan of using the hop-on/hop-off bus tours for a quick excursion around a city. I have used them on my first visit to a number of cities, including Dublin. I also like the Guinness Storehouse tour for several reasons, especially for a time-constrained visit because you can do it self directed--you can go there for 20 minutes or two hours without committing to a tour guide.

And even if you don't like Guinness or beer, you get a very good feel for Dublin's history from some of the exhibits since Guinness has been part of Dublin for nearly 400 years. In addition, the bar at the top of the Storehouse has great views of the city and the Wicklow Mountains beyond.

Back on the hop-on/hop-off, find a place for a quick bite, and you can head back to the airport in time for your flight.

if you decide you don't have time to head into the city centre ... you could buy a month's membership of ALSAA and go or a workout / swim [timetable permitting]

https://maps.app.goo.gl/BqCbQam4KVHYXmFK9