Dublin sits between mountains and the sea with the richest concentration of prehistoric monuments in Europe an hour north of it. The day trips are genuinely better than the city’s second-tier attractions, and several of them cost the price of a train ticket.
Within an hour, by public transport
Howth — 30 minutes on the DART. A fishing village on a rocky headland with a cliff loop, seals in the harbour, and the best seafood near Dublin. The easiest good day out from the city and the cheapest.
Dún Laoghaire and Sandycove — 20 minutes on the DART south. Two Victorian piers to walk, the James Joyce Tower at Sandycove where Ulysses opens, which is free, and the Forty Foot, a rocky sea-bathing place where Dubliners swim year-round including Christmas morning. Bring a towel; people will assume you are getting in.
Malahide — 25 minutes on the DART north. A castle occupied by one family for 800 years, walled gardens, and a village with good restaurants.
Bray to Greystones cliff walk — DART to Bray, then a 7km coastal path around Bray Head to Greystones, and the DART home from there. Two hours of walking with the sea below you the whole way. Check the path is open; sections have closed for erosion.
The two that need a bus or a car
Glendalough — 90 minutes by St Kevin’s Bus from St Stephen’s Green. A sixth-century monastic city with an intact round tower, in a glacial valley with two lakes and a network of marked walks. The single best day trip from Dublin. Check the return bus times before you go; there are not many.
Brú na Bóinne — Newgrange and Knowth — about an hour north in County Meath. A Neolithic passage tomb complex from around 3200 BC, which makes it older than Stonehenge and older than the Egyptian pyramids. Newgrange’s passage is aligned so that at the winter solstice the rising sun runs the length of it and lights the chamber; there is an annual lottery for places, with thousands of applicants for a handful of spots.
Access is only through the visitor centre and only on a guided shuttle — you cannot drive to the mound. Book ahead in summer. Day tours from Dublin cover it, often with the Hill of Tara.
Further, and still a day
Kilkenny — 90 minutes by train. A medieval city with a castle, a cathedral, a round tower you can climb, and the best-preserved medieval street pattern in Ireland.
Belfast — two hours on the Enterprise train from Connolly, and it is a different country with a different currency. The Titanic Quarter, the political murals and the black taxi tours. Sterling, not euro; no border checks.
The Wicklow Mountains — the Sally Gap and Glenmacnass by car, which is one of the great drives in Ireland and needs a car to be worth it.
Powerscourt Estate, near Enniskerry — formal gardens, a Palladian house and Ireland’s highest waterfall a few kilometres away.
Trim Castle in Meath, the largest Norman castle in Ireland, used for Braveheart.
What is not a day trip
The Cliffs of Moher and the Giant’s Causeway are both sold as Dublin day trips and both involve seven to eight hours on a bus for ninety minutes at the destination. They are magnificent and they deserve an overnight. If you only have a day, Glendalough is a better use of it.
Galway and the west are three hours each way. Doable, and a poor idea.
Practical
The Leap Card covers the DART and commuter rail as well as the city network.
Weather. The Wicklow mountains make their own, and a fine morning in Dublin means nothing at Glendalough. Layers and waterproofs, and boots for anything off tarmac.
Sundays have reduced bus services on the rural routes, which is exactly when people want them.
Tours from Dublin are a reasonable option for Newgrange, Glendalough and the Wicklow drive, where public transport is thin. For anything on the DART line, do it yourself for a tenth of the price.
Building a day around one
A workable shape for any of these: leave after breakfast rather than at dawn, because the light is better later and the coach parties clear by mid-afternoon. Take food — the villages have one café and it will be full. Check the last bus or train before you commit to a walk, because rural Irish services thin out sharply after about six and the taxi fare back from Wicklow is memorable.
Which to choose
One day and no car: Howth. It is thirty minutes, it costs nothing beyond the fare, and the cliff loop plus a fish lunch is a complete day.
One day and you want the thing you will remember: Glendalough.
Interested in prehistory: Newgrange, booked ahead.
Travelling with children: Malahide Castle, or the Bray to Greystones walk with ice cream at the end.
A second city: Kilkenny by train, or Belfast if the political history interests you.