The lounges in DUB are pretty basic, even the one owned by EI. You should envisage an Admirals Club or United Club but smaller with less amenities. The 51st & Green lounge, beyond US immigration, is apparently better but has a nominal two hour limit (in practical terms this works anyway because you won't go down to pre-clearance before that anyway), but also a two drink one.

Six hours in the airport, with or without lounge, will merely bore you to death, so even if you opt for the long layover my advice would be to get out and at least see something. The good news is that's perfectly possible. There are regular buses to Swords which take about 10 minutes, and the castle there has free entry. You'd have plenty of time to do this. Further north is Malahide, where you get a bigger castle (charge) and coastline but you may be clock-watching a bit.

You do just have time to venture into Dublin on the layover, but you'd need to be selective on what you see. Public bus takes about 45 minutes each way including wait time and you probably want to be back at the airport two hours before departure at the latest. So, that gives you around two hours in central Dublin including clearing immigration. That is enough time to see at least some of Grafton Street, Temple Bar, St Stephens Green, Trinity College, the Mollie Malone statue and - rather more culturally - possibly the Book of Kells if you are confident enough to pre-book so you don't have to queue for admission and prioritise it. Alternatively you could visit one of the distilleries (I'd recommend Teeling) or the Guinness Museum in The Liberities, although that would probably be all you'd have time for if you made that choice. The Irish Whiskey museum is a viable central alternative if you want to learn about distilling and is certainty better than the normal tourist-trap model for those things.

Buses will take cash if you don't want to buy a Leap card from the newsagent at the airport, but it's exact money only - so you'll need Euro coins. You can buy Leap cards from the newsagent in the tunnel leading from terminal to bus stops and top them up from an App if you need more money than is initially loaded (you probably won't).

An overnight would make this much easier, of course. Accommodation can be extremely expensive in Dublin, but there are some central Premier Inns which usually undercut the hotel chains and most B&Bs.

For security, in J with EI you'll get Fast Track for the first check which will get you airside. After a sticky period last year, it's usually a fairly painless and quick experience now. The second USA check operates by their rules.