The early history of the county is obscure and frequently confusing with various tribes seeking control. Christianity came to Ireland at the start of the fifth century, if not earlier, and brought about many changes, including the introduction of writing and reading. St. Patrick, Ireland's national apostle, whose floruit was the fifth century, is chiefly credited with the conversion of the pagan Gaels. Recent research indicates that St. Patrick spent considerable time in County Mayo, where according to tradition and some written sources he spent forty days and nights on the summit of Croagh Patrick fasting and praying for the people of Ireland; and had associations with places like Aghagower near Westport, Ballintubber (well-known nowadays for its medieval abbey which has remained in continuous use through all vicissitudes from its foundation in 1216); and Foghill near Killala, which has been identified by some writers with the Silva Vocluti , 'the wood of Fochluth beside the western sea' mentioned by Patrick himself in his Confessio.
From the middle of the sixth century onwards, hundreds of small monastic settlements were established around the country, many of which became very important. Some examples of well-known early monastic sites in Mayo include Mayo itself near Balla, Aughagower, Inishmaine, Ballintubber, Errew, Kilmore Erris, Balla, Cong, Killala, Turlough, Moyne near Cross, and island settlements off the Mullet peninsula like Inishkea North, Inishkea South and Duvillaun More.
One of the most interesting monastic sites in Co. Mayo was that from which the county derives its name - Maigh Eo. Colmán of Lindisfarne, having been defeated by the 'Romanist' party at the synod of Whitby (in Northumbria, in the north-east of England) in 663, withdrew with his followers, via Iona, to Inishbofin off the west coast of Galway. As a result of disagreement between the Irish and the English monks in the little community, the latter moved to the 'plain of yews', about sixteen kilometres south-east of the present town of Castlebar. The monastery they established there, known as Mag nÉo na Sachsan ('of the Saxons'), became renowned as a centre of learning, and continued to attract monks of English birth for a century and more after its foundation.
It is an indication of Mayo's importance in the middle ages that, when, in 1152, the synod of Kells introduced a system of diocesan organisation to the Irish church, one of the dioceses established west of the Shannon was that of Mayo. In the aftermath of the Reformation, the Established Church united the see to Tuam. The Catholic diocese was finally absorbed by Tuam, by papal decree, some time after 1631. The monastery at Mayo became a collegiate church sometime early in the 13th century, and about 1370 it became an abbey (St. Michael's) of Augustinian Canons. It survived until the dissolution of the monasteries after the Reformation. It will be clear from the foregoing that 'Mayo' as the name of the abbey and, more importantly, of the diocese, was very much in circulation around 1570, when it came to naming the new county established by Sir Henry Sidney.
The Vikings or Norsemen first attacked Ireland in 795 and Mayo around the start of the ninth century. On arrival, they started to plunder and loot places of wealth especially monasteries. It was partly in response to those attacks that round towers were later erected in monastic enclosures (most were erected in the 12 century). There are about 65 of these fine structures surviving in Ireland, with five located in County Mayo: Aughagower, Balla, Killala, Turlough and Meelock. The Viking invasion led to the establishment of settlements in a number of locations like Dublin, Cork, Wexford and Waterford which later developed into towns and cities.
The Anglo-Norman colonisation of Ireland from1169 onwards was one of the most significant events in the development of Ireland. Mayo came under Norman control in 1235. The Norman conquest meant the eclipse of many Gaelic lords and chieftains, chiefly the O'Connors of Connacht, but the invaders soon adopted Gaelic customs and began to marry with the native Irish and became as the phrase has it: 'more Irish than the Irish themselves'. This process of Gaelicisation is best exemplified in the adoption by various Norman families and branches of families of new surnames based on Gaelic-style patronymics. Examples of Mayo surnames today with Norman origins include Barrett, Burke and Bourke, Costello, Culkin, Davitt, Fitzmaurice, Gibbons, Jennings, Joyce, McEvilly, Nally, Padden, Staunton and Walsh. The Normans started numerous towns and developed some existing settlements into towns, as well as organising fairs and markets. They developed roads, bridges, sea-ports and promoted the growth of trade both domestic and foreign as well as improving the agricultural methods then in vogue.
A noteworthy feature of the period with which we have been dealing was the buildings of abbeys or friaries for the new mendicant orders - Augustinians, Carmelites, Dominicans and Franciscans - principally by the Hiberno-Norman families. A number of early monastic sites - such as Cong, Inishmaine, Ballintubber, Errew, and Mayo - had been chosen as locations for abbeys of the Augustinian Canons Regular, built under the patronage of Gaelic families (particularly the O'Connors) in the 12th and 13th centuries.
The first friary founded under Norman auspices in Mayo was that of Straide (alias Strade) established for the Franciscans by Jordan de Exeter, probably between 1240 and 1250. It was very soon (in 1252) transferred to the Dominicans. Another Dominican house, also thought to have been founded by a de Exeter, was Rathfran, dating from 1274. The Prendergasts founded Ballinasmalla, near Claremorris, for the Carmelites around 1288. Another Carmelite foundation, dating from 1298, was Burriscarra, which was built by the Stauntons. Abandoned after about eighty years by the Carmelites it was later occupied by the Augustinian friars. The Augustinians were given a house in Ballinrobe around 1313, by one of the de Burgos. No other notable foundation is recorded for over a century, until about 1430, when the Mac Costellos established the Dominicans in Urlaur and the Augustinians in Ballyhaunis. A decade later Rosserk Friary was founded for the Franciscan Third Order by one Joye (or Joyce). Nearby Moyne Friary was built for the Franciscan friars by Mac Uilliam ochtarach (de Burgo)around 1455, while, a couple of years later, the only Gaelic foundation of the period, Murrisk, in the shadow of Croagh Patrick, was established for the Augustinians by Tadhg Máille, the local chieftain. The latest foundation of any significance was the Dominican Friary of Burrishoole, built around 1469 by Mac Uilliam ochtarach, Richard de Burgo of Turlough.
Almost all the foundations mentioned above were suppressed in the wake of the Reformation in the 16th century. One or two have been rebuilt and restored, but in most cases, only the ruins survive, pleasing, if poignant, late Gothic relics of what must have been among the most striking buildings in the countryside of pre-Tudor Ireland.
The 15th century was marked by frequent quarrels between the Mayo Burkes (as the people of Mac Uilliam ochtair may be called for convenience) and the Clanrickard Burkes of what is now County Galway, as well as by much internecine fighting among the minor Norman lords of Mayo. From mid-century onwards, the O'Donnells, the great Gaelic lords of Tír Chonaill (in present-day Co. Donegal), interfered frequently in the affairs of north Connacht, as they sought to extend their way southwards. They met with opposition from the Burkes, who were also quite often embroiled in the affairs of their eastern neighbours, the O'Connors of Roscommon and Sligo. Another Gaelic family, the O'Kellys of east Galway and south Roscommon were usually to be found in alliance with the Burkes of Mayo.
The turn of the century saw the Lord Deputy, Garrett Mór Fitzgerald, the great Earl of Kidare, ruling as virtual king of Ireland. In August 1504 he demonstrated his power by inflicting a crushing defeat on his son-in-law, Ulick Burke, Earl of Clanrickard, in a battle at Cnoc Tuagh (Knockdoe) near Galway. Among those who joined the great alliance against Clanrickard an his Munster allies were his cousins and rivals, the Mayo Burkes.
A mere thirty years after Cnoc Tuagh the great House of Kildare succumbed to the growing might of the Tudor monarchy, and by mid-century English power was making itself felt in Connacht, where the rivalry between the Mayo and Clanrickard Burkes had flared up again into war. By the late 1560s the Lord Deputy, Sir Henry Sidney, had procured the submission of both de Burgo lords, and was making provision for the future government of the province in the interests of the Crown. In July 1569 Sir Edward Fitton was appointed President, or Governor, of Connacht. One of the first tasks facing him and his council was to lay down the boundaries of the new counties of Connacht and Thomond. Almost immediately he was faced with what was to become a commonplace over the next thirty years - a rebellion by the Mayo Burkes. Fitton, with various allies, including Clanrickard, met them in battle at Shrule in June 1570. The outcome of the battle was somewhat indecisive, but Mac Uilliam ochtair submitted and made peace shortly afterwards. 1572 saw another short-lived revolt, this time in alliance with two sons of Clanrickard. When Clanrickard's sons rebelled again in 1576, however, the Mayo Burkes remained loyal, holding Castlebar for the Queen.
It was in this last campaign, in 1576, that the remarkable 'sea-queen' from the shores of Clew Bay, Gráinne Ní Mháille (variously anglicised Granie ny Maille, Grace O'Malley, Granuaile, etc.) first makes her appearance in history, offering the services of her galleys and two hundred fighting men to Lord Deputy Sidney. But within two years Gráinne's second husband, Risteard an Iarainn - a Burke, and claimant to the MacWilliamship - was in revolt; his rebellion simmered on until 1582, when the new Lord Deputy, Sir Nicholas Malbie, recognised him as MacWilliam, and later knighted him.
A rebellion in 1585 by various branches of the Burkes was suppressed with great severity by the new Governor of Connacht, Sir Richard Bingham. A month later, a force of 2,000 Scots mercenaries came to Connacht to assist the Burkes, but they were routed with a great slaughter near Ardnaree.
In the summer of 1588 the galleons of the Spanish Armada were wrecked by a storm along the west coast of Ireland. Some of the hapless Spaniards came ashore in Mayo, only to be robbed and imprisoned, and in many cases slaughtered.