Can I leave the airport on a layover in Dublin?

Six hours is the honest floor for a Dublin layover. Immigration runs 15-25 minutes at Dublin Airport for EU and EEA e-gate holders, closer to 30-40 minutes in the manual queue during morning peaks. The Airlink 747 bus from the terminal to O'Connell Street takes about half an hour and costs €13 return. A taxi runs €25-35 and might shave 5 minutes off the trip. You want 90 minutes back at the airport before a long-haul departure, so a 6-hour window leaves you about 2 hours in the city center. Under 5 hours, stay airside. Terminal 2 has decent coffee at Butlers Chocolate Café and free WiFi throughout both terminals.

Two hours on foot covers a tight loop between Trinity College and Christ Church Cathedral. Get off the 747 at O'Connell Street, cross the Ha'penny Bridge (the iron footbridge creaks underfoot, and buskers tend to set up at the south end), and walk 8 minutes south to Trinity's Front Square. The Book of Kells exhibition opens at 09:30 and takes about 30 minutes. From Trinity, it's a 10-minute walk along Dame Street to Dublin Castle, built on the orders of King John in 1204. The State Apartments are open daily and rarely crowded before 11:00. Three minutes uphill from the castle, Christ Church Cathedral has stood since 1031. The Chester Beatty Library, inside the castle grounds, charges no entry fee. Solo travelers have an edge on this kind of run. No coordinating. No waiting for someone in the gift shop. You set the pace.

Dublin scores 6.8 out of 10 on TTDI's solo-traveler safety index (see the TTDI Solo-Safety Index). The city center feels safe during daylight and well into the evening, with foot traffic along Grafton Street and through the Temple Bar area until late. After dark, O'Connell Street's north end and the boardwalk along the Liffey can feel rougher. I'd walk south of the river at night without hesitation, but north of the Liffey past about 23:00 I'd stick to well-lit roads or take a taxi. The last Airlink 747 back to the airport leaves O'Connell Street around 23:30. If your layover stretches past dinner, a solo seat at the counter of The Winding Stair on Ormond Quay is one of the better meals in Dublin. No reservation needed for counter seats. The menu runs seasonal Irish cooking, and a main costs €22-28. You'll smell brown butter and hear the Liffey below the window.

Luggage storage at Dublin Airport costs €8-10 per bag per day at the left-luggage desk in T1 arrivals. Use it. The cobblestones in Temple Bar will punish a wheeled carry-on, and the uneven granite costs you 10 minutes you don't have on a tight window. Late August weather in Dublin currently sits around 18-19°C with 82% humidity, so pack a light rain jacket even if the sky looks clear when you land. Dublin's rain tends to arrive in 20-minute bursts, not all-day downpours, so you won't lose your whole layover to it. Ireland uses the euro, and at today's rate 1 USD buys about €0.86. Card payment works everywhere in the center, including on the buses, so you don't need to hunt for an ATM.