County Roscommon (Contae Ros Comáin) is in West Ireland, historically part of the Kingdom of Connacht. It's low-lying and rural, with the River Shannon forming its eastern boundary.
As the county is quite narrow, towns on or just across its boundary may be good bases for exploring.
Little is known about the pre-Celtic peoples of this area, but the Coggalbeg hoard demonstrates their artistry. It was found in 1945 near Strokestown by a turf-cutter, and comprises a gold lunula (crescent moon shape) and two gold discs, all lightweight and suitable for personal ornament. The hoard was tucked away in a pharmacist's safe, until 2009 when the safe was stolen during a break-in. The thieves were only interested in ready cash and dumped the other contents into a skip in Dublin, but were spotted on CCTV. The police retrieved the hoard, which is now on display in the National Museum in Dublin and dated to 2000 BC.
By the early centuries AD the west of Ireland was populated by Celts in the Kingdom of Connacht, ruled from Cruachan, nowadays called Rathcroghan and with extensive remains. The area became christianized, and Coman Mac Faelchon built a monastery in the 5th century. The woods near the monastery became known as Ros Comáin, "Coman's Wood", hence Roscommon town. The native rulers were subjugated by King John in the 13th century, and the Tudors set out the county boundaries.
County Roscommon has low hills, mostly in the northeast towards Lough Allen, with Seltannasaggart reaching 428 m (1404 ft). The farmland is poor and drains east into the River Shannon and its loughs, which are navigable and form the natural boundary with Counties Leitrim and Longford. There's been little industry since a 17th / 18th century spell of coal and iron mining, and thus little to foster population growth or urban development. "Unspoilt" is how the County Tourist Agency pitches it. The region is collectively promoted under the brand "Ireland's Hidden Heartlands".
Two railways cross the county. Trains from Dublin Connolly run via Drumcondra, Maynooth, Mullingar, Longford and Dromod to Carrick-on-Shannon and Boyle, and continue to Ballymote and Sligo. Trains from Dublin Heuston run via Kildare, Portarlington, Tullamore and Clara to Athlone, whence they either continue west to Athenry and Galway, or head northwest via Roscommon and Castlerea to Castlebar and Westport.
Bus Éireann buses run from Dublin Busáras and Airport to Carrick and Boyle, heading to Sligo, and to Athlone heading for Galway. Roscommon town has no direct service, train is better, but you could change at Athlone for the bus to Roscommon, Castlebar and Westport.
With your own car, from Dublin take M4 to Kinnegad. For the south of the county, stay on it (becoming M6) to Athlone, then take N61 for Roscommon. For the north, exit onto N4 through Mullingar and Longford; stay on N4 for Carrick and Boyle, but take N5 for Strokestown.
The train will get you between Carrick and Boyle, and between Athlone, Roscommon and Castlerea.
Local Link buses run 2 or 3 times a day between Roscommon, Strokestown, Tulsk, Elphin, Boyle, Lough Key and Arigna.
The countryside is low-lying and the distances in such a small county are not great, so bicycle is a viable option.
And then the rain sets in, what a surprise. Thank heavens you hired a car from Dublin Airport.
Roscommon has the best range, but the grand splurge is Kilronan Castle, see Drumshanbo.
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