For the overnight on the way in, since you are only having to clear one security line (vs. two on the way back for pre-clearance), I would absolutely not spend 6 hours in the lounge. I flew J on AA, but the only semi-time consuming aspect is if it's an incredibly busy travel time and you have to go through both security lines (and the US part was the longest for me, but I have only returned once from DUB, so N=1 on that). Even doing that, having arrived three hours early due to predicted summer crowds (accurately predicted), I had time for over an hour in the lounge prior to walking a bit to my gate and still waiting. I did not need the extra time--almost too much.

If you've never been to Ireland, by all means, do the overnight and see a small part of Dublin. DUB is not worth the lounge access in that case. Pick something you really want to see in the city and/or just hop a bus and explore.