The monastery of Mohill-Manchan () was anciently located at Mohill, in county Leitrim. The earliest church was founded by Manchán of Mohill in the 6th century. Little is known about the former monastic community here. About the year 1216, the monastery became a religious house of the Canons Regular of Saint Augustine dedicated to the Saint Mary until suppression . The Priory of Mohill was briefly revived during Confederate Ireland rule but suppressed again by . From the ruins St. Mary's Church, Mohill, of Protestant denomination, was established in the 18th century.
The former monastic settlement of Mohill was one of a multitude of monasteries that sprang up during 6th century Ireland. A Christian missionary named Manchan founded a church here between AD. Whether or not Manchan died at Mohill is unknown, though his remains may have been preserved and enshrined here.
Nothing is known of the monastic community at Mohill, but it surely consisted of a church, school, mill, house of hospitality, Christian burial ground, monastic cells, "house of tears", and a round tower. The monastery was governed by the bishop, abbot, and a uniquely Irish "erenagh", power being exercised by one person, or perhaps combined in practical permutations. It was strongly bound to the túath of Muintir Eolais, allowing the Bishop little influence; the lifestyle followed asceticism.
The monastery of Mohill was of considerable extent stretching across the river to encompass the townlands of Mohill, Tullybradan, Lisdadanan, Coolabaun, Cappagh, Gortfadda, Drumcroy and Drumkilla, an estate of acres. A daughter church at Cloonmorris was founded AD.
From the 13th century () following a reform movement of the Irish church, the monastery became the Priory of Saint Mary's of Mohill-Manchan. The priory belonged in the diocese of Ardagh, with Canons regular adhering to the Rule of St. Augustine. The Priory of St. Mary's was headed by a prior, their office being valued at , or in the 15th century. In 1418, the Priory of St. Mary's at Mohill was described as "conventual, with cure", and a dependency of the priory of Abbeyshrule in Ardagh diocese, and sufficiently attractive for a canon from Clonmacnoise to enter its doors. In 1475, the Priory of St. Mary's Mohill was described as "conventual", "elective", "with cure", whose value did not exceed , and "not depending on any other monastery or place", though a petition dated 1477 again alleged the dependency existed
It is impossible to fully catalogue the succession of holders of abbots, priors, and canons at this ancient monastery. The information below is preserved.
The Irish Annals refer to the priory of Mohill many times. For 1430AD the Annals of the Four Masters highlights an interesting privilege was the right of Mohill Priory to offer sanctuary to a fugitive fleeing from enemies. The Annals also records a military invasion of Muintir Eolais in March 1590, when the local Túaths were defeated and Mohill Priory was forfeited to the English crown.
From 1540 the English were forcefully suppressing Monasteries throughout Ireland, and "the end of the sixteenth century [was] a bad time for priests and papists". Some sources suggest the monastery was suppressed as late as 1621, however Mohill was described as the "Queens's manor" consisting of 'about six quarters' in , and "an immense" english army occupied Mohill and routed the "Muintir Eolais" chieftains.
The "late monastery of Moghill" was divided between Edward Barrett , and Terence O'Byrne of Muintir Eolais . Barret's parcel subsequently passing to an englishman named John Crofton , who bequeathed same to his son Henry by a deed dated . On dissolution, the Priory possessions were the townlands of Tullybradan, Coolabaun, Gortfadda, and Drumcroy, totaling , a church, two stone buildings, and the cemetery.
Confederate Ireland rule was established , and during the Eleven Years' War the Priory of Mohill was re-established in some manner. Ecclesiastical documents record Canon Antony Mag Raghnaill, otherwise Reynolds, being appointed Prior of Maothail-Manchain on 21 August 1648.
The priory was suppressed again during the devastating Cromwellian conquest of Ireland .
Priory clergy were expelled and risked execution under the Penal Laws. In 1666, four Reynolds priests (James, Loghlin, Richard, Walter) are among "forty nine Catholics from hiding places in the woods" in County Roscommon, who signed a letter in support of the Pope and protesting the loss of their 'due liberties'. And in 1713 an elderly Father Connor Reynolds "of Jamestown in the county of Leitrim" who had been exiled in Spain since 1681, was captured hiding in a trunk on a fishing boat arriving at Dungarvan port and imprisoned at Waterford jail.
Today nothing survives of the early ecclesiastical site here, except for an inaccessible old school-house, and the base of a round tower located near the old persons home in the town. The remains of the abbey or sanctuary forms the south, and east, walls of the Hyde family vault in the graveyard of 'Saint Mary's church' later built on the Priory ruins. The Croftons also built an old Castle at Rinn Lough from the ruins of Mohill Priory, as small free stones of the same class were evidenced in each. There may be other undocumented ruins on the wider monastic site, though it's not classed as a national monument or heritage site.