OFFALY COUNTY, a county of Ireland in the province of Leinster, bounded north by Meath and Westmeath, west by Roscommon, Galway and Tipperary (the boundary with the first two counties being the river Shannon) ; south by Tipperary and Leix County, and east by Kildare. The area is 493,999 acres or about 772 sq.m. Pop. (1926) 52,521. The greater part of the county is included in the Carboniferous Limestone plain of central Ireland. In the south-east the Slieve Bloom Mountains, composed largely of Old Red Sandstone, form the boundary be tween Offaly County and Leix County, and run into the former county from south-west to north-east for a distance of about 20 m. consisting of precipitous crags through which there are two narrow passes, the Black Gap and the Gap of Glandine. In the north-east, basic volcanic rocks rise in Croghan Hill to a height over 700 ft. In the centre of the county from east to west a large portion is occupied by the Bog of Allen. Here and there drier deposits of esker-gravels rise as green hills above the clay soils and bogs of the plain. The county shares in the advantage of the navigation of the Shannon, which skirts its western side. The Brosna, which issues from Loch Ennell in Westmeath, enters the county near the town of Clara, and joins the Shannon after re ceiving the Clodagh and the Broughill. A small portion of the north-eastern extremity is skirted by the upper Boyne. The Bar row forms the south-eastern boundary with Leix. The Little Brosna, which rises in the Slieve Bloom Mountains, forms the boundary of Offaly County with Tipperary, and flows into the Shannon.
Offaly County, with portions of Tipperary, Leix County and Kildare, at an early period formed one kingdom under the name of Offaly, a title which it retained after the landing of the English. Subsequently it was known as Glenmallery, Western Glenmallery corresponding closely to the present Offaly County, and Eastern Glenmallery to Leix County. These two divisions were formed into shires in 1556, being then known as King's County and Queen's County respectively.
In the Slieve Bloom Mountains is a pyramid of white stones called the Temple of the Sun or the White Obelisk. There are many Danish raths, and a chain of moats commanding the passes of the bogs extended throughout the county. On the borders of Tipperary is an ancient causeway leading presumably to a crannog or lake-dwelling. The most important ecclesiastical ruins are those
of the seven churches of Clonmacnoise (q.v.) on the Shannon in the north-west of the county, where an abbey was founded by St. Kieran in 648, and where the remains include those of churches, two round towers, crosses, inscribed stones and a castle. Other famous religious houses were Durrow Abbey, founded by St. Columba in Monasteroris founded in the 14th century by John Ber mingham, earl of Louth; and Seirkyran Abbey, founded in the beginning of the 5th century. The principal old castles are Rath more, probably the most ancient in the county; Banagher, com manding an important pass on the Shannon; Leap Castle, in the Slieve Bloom Mountains; and Birr or Parsonstown.
The whole of the county would appear to have been covered formerly by a vast forest, and the district bordering on Tipperary is still richly wooded. The soil is generally either a deep bog or a shallow gravelly loam. On the borders of the Slieve Bloom Mountains there are some very rich pastures, and there are also extensive grazing districts on the borders of Westmeath, chiefly occupied by sheep. Along the banks of the Shannon there is good meadow land. With the exception of the tract occupied by the Bog of Allen, the county is nearly all under tillage, the most pro ductive portion being that to the north-west of the Hill of Crog han. Oats, barley and rye, potatoes and turnips, are all consider ably grown ; wheat is almost neglected, and the acreage of all crops has a decreasing tendency. Cattle, sheep, pigs and poultry are bred increasingly; dairies are numerous in the north of the county, and the sheep are pastured chiefly in the hilly districts.
The county is traversed from south-east to north-west by the Portarlington, Tullamore, Clara and Athlone line of the Great Southern railway, with a branch from Clara to Banagher; other branches run from Roscrea (Co. Tipperary), to Parsonstown (Birr), from Enfield (Co. Kildare) to Edenderry, and from Streamstown (Co. Westmeath) to Clara. The Grand Canal runs through the county from east to west, entering the Shannon at Shannon harbour.
The administrative counties of Leix and Offaly together return 5 members to Dail Eireann.