Walk into the quadrangle and count the four D-shaped corner towers

Find the twin gate towers and the one that still holds its vaulted roof

Find out how many times the castle changed hands between the crown and the O'Conors

Walk the town and find the façade of the old gaol — all that was kept when the building behind it came down — and the Dominican friary ruins

Robert de Ufford, Justiciar of Ireland, built Roscommon Castle in 1269 on land he had seized from the Augustinian priory — a quadrangular fortress with four D-shaped corner towers three storeys high and twin towers at the gate, one of which still has its immensely sturdy vaulted roof. It changed hands repeatedly between the crown and the O'Conors, and it now stands roofless on a green on the edge of the town, free and unfenced, with people walking dogs round it.

Roscommon Castle: Four D-Shaped Towers on a Green

Find Fedlim Ó Conchobair's effigial slab in the niche in the north-east corner

Look at the carved figures on the tomb panel beneath the effigy

Work out what it meant for a Gaelic king to found a Dominican house in 1253

Walk between the abbey and the castle at opposite ends of the town — the castle went up in 1269, four years after Fedlim was buried

Fedlim Ó Conchobair, King of Connacht, founded a Dominican priory at Roscommon in 1253, and when he died in 1265 he was buried in it. The effigial slab that covered his tomb is still there, set in a niche in the north-east corner of the church — a carved medieval king lying in a roofless ruin in the middle of an ordinary town, with the castle he would have known at the other end of it.

Roscommon Abbey: The O'Conor King's Tomb

Walk among the ruins of the churches of Diarmaid's monastery, founded in 560

Find the place on the shore where legend says Medb was killed

Look back across the lough to the Longford and Roscommon shores

Inchcleraun — Inis Cloithrinn, Clothru's island — is a 143-acre island in the middle of Lough Ree, and Diarmaid the Just founded a monastery on it in 560. Between 800 and 1300 its churches were plundered and burned again and again, and they stand in ruins. Legend says Queen Medb retired here and was killed on the shore by a stone — or in another version a lump of cheese — slung from the Longford side; one point is still called Grianan Meva, Medb's sunny place. Its other name, Quaker Island, comes from a Quaker who once owned it.

Inchcleraun: Diarmaid's Monastery on Lough Ree

Look out towards Inchcleraun and its early Christian church ruins

Walk the Shannon bank where the river enters the lough

Fish the lough — it is one of the best coarse fisheries on the Shannon

Lough Ree — Loch Rí, the king's lake — is the second of the three great lakes of the Shannon and the second largest after Lough Derg, and Lanesborough sits at the northern end of it where the river comes in. Out in the northern part of the lake lies Inchcleraun, with the remains of several ancient churches from a monastery founded in the early Christian era. The lough is wide enough to have its own weather and its own long-running monster stories.

Lanesborough and Lough Ree: The Second Lake of the Shannon

Visit the Rathcroghan Visitor Centre in Tulsk, gateway to a 240-site royal complex, seat of the legendary Connacht queen Medb

Book a guided tour from the Rathcroghan Visitor Centre out to Oweynagat, the Cave of the Cats, said in medieval Irish literature to be the dwelling of the Morrígan

Walk the fields around Rathcroghan mound itself, still unexcavated after more than a century of study

Find the cave's Samhain associations explained at the visitor centre, the festival now echoed in modern Halloween

Rathcroghan is the royal complex of Connacht — 240 archaeological sites spread across ordinary Roscommon farmland, with the seat of the legendary queen Medb at the centre of it and the main mound still unexcavated after a century of study. The reason to come is Oweynagat, the Cave of the Cats: a natural limestone cave with an early medieval drystone souterrain built over its entrance, its lintels carrying ogham inscriptions, which medieval Irish literature names as the dwelling of the Morrígan and as an entrance to the Otherworld. It was associated with Samhain, the festival that modern Halloween came out of.

Rathcroghan: The Cave of the Cats

In the Famine Museum, find the estate papers themselves — the rent rolls, eviction notices and letters Denis Mahon generated in 1846 and 1847, originals rather than copies

In the house kitchen, look up at the gallery running along the wall above, built so the family could watch the servants at work without going down among them

Read the account of Denis Mahon's assassination in the museum — ambushed in his carriage on the road home from a relief meeting on 2 November 1847

Walk the six-acre walled garden and its herbaceous border, one of the longest in Ireland

Strokestown Park House in Roscommon is a Palladian mansion built for the Mahon family in the 1730s. During the Great Famine, Major Denis Mahon evicted some 3,000 people from the estate in 1847 and shipped 1,490 of them to Quebec on overcrowded "coffin ships" — many died on the way or soon after landing. On 2 November 1847 he was ambushed and shot dead in his carriage on the road home from a famine relief meeting. The archive of letters, rent rolls and eviction orders he left behind survived intact and is the core of the Irish National Famine Museum on the grounds.

Strokestown Park: The Landlord's Archive

See the 18-metre preserved stretch of oak trackway in the humidified hall

Find out how dendrochronology dated the timbers to 148–147 BC, contemporary with the fall of Carthage

Walk the bog outside and picture the quicksand and ponds the road was built to cross

Work out why a road this wide was built here at all, in a place with nothing obvious at either end

In 1984 timbers pulled out of a Longford bog turned out to be much later than anyone had assumed — Iron Age, not Bronze Age. Dendrochronology dated the oak to late 148 or early 147 BC — the same years Rome was destroying Carthage — and what they had found was a corduroy road of split planks laid on raised rails, wide and solid enough for wheeled traffic, running at least a kilometre across what was then bog, quicksand and ponds. Locals had always called it the Danes' Road. An 18-metre stretch is preserved indoors in a humidified hall, because exposed to air the oak would simply crumble.

The Corlea Trackway: An Iron Age Road Through the Bog

Tour Clonalis House, still lived in by the O'Conor Don

Find out what is in the family archive of around 100,000 volumes

Trace the descent from Cathal Crobhdearg Ua Conchobair, King of Connacht

The O'Conor Don are direct descendants of Cathal Crobhdearg Ua Conchobair, King of Connacht — one of the very few Gaelic royal lines whose descendants still live on their own land. Charles Owen O'Conor built the present house at Castlerea in 1878 to replace an earlier one that kept flooding. What makes it worth the visit is not the architecture but the archive: around 100,000 volumes and documents held by the family, which makes a private Roscommon house one of the more valuable repositories of Irish history anywhere.

Clonalis: The House of the O'Conor Don

Drive the lake roads north-west of the town and count how many loughs you pass

Walk in Killykeen Forest Park among the drumlins and the water

Find out why the ice left Cavan with this many small hills and this much water

Fish or paddle on one of the loughs — most of them have no one on them

Cavan is the county of the drumlins and the lakes, and the local boast is that there is one lake for every day of the year. The truth is close enough: the ice left a field of small rounded hills, the water filled every hollow between them, and the result is a landscape where the road bends constantly because it has no choice. Lough Oughter is really one continuous flooded maze rather than a lake, and the town sits in the middle of it all.