The pattern of land ownership in Mayo underwent a continuous if slow metamorphasis in succeeding generations as clans evolved and grew stronger or were eclipsed by their neighbours and London administrations began to play a more significant and direct role in the affairs of Ireland. The traumatic Cromwellian settlement which commenced in 1641 ended a decade later with a stern regime in absolute control of the country and grimly determined to reward its friends and punish its enemies. The most significant feature of the "Cromwellian settlement" as it is known, was the plan to repay Commonwealth soldiers and adventurers for their services with grants of land in ten Irish counties. The landowners displaced as a consequence of implementing this scheme were, if found to be innocent of participation in "the late rebellion", to be given lands, in proportion to their original estates, in four counties west of the Shannon - Mayo, Galway, Roscommon and Clare. The "transplantation to Connacht" also involved transplantation within Connacht, as existing landowners west of the Shannon, displaced to make way for the new arrivals, had to be found estates elsewhere in the Province.
For the vast majority of people in County Mayo the eighteenth century was a period of unrelieved misery, with some minor famines. Because of the operation of what were called 'the penal laws', Catholics had no hope of social advancement while they remained in their native land. However, emigration could and did lead to new opportunities and challenges for many like William Brown (1777-1857), who left Foxford at the age of nine and thirty years later was an admiral in the fledgling Argentine Navy. Today he is revered as 'the father of the Argentine Navy', and as a national hero in that country.
Culturally, 16th century Mayo made some contribution to the "hidden Ireland" of the time, and two Mayo-born poets from the period have retained considerable popularity: Riocard Bairéad (d. 1819) from the Mullet, whose songs included 'Eoghan Cóir', 'Preab san l', and 'Tarraingt na Móna' and blind Anthony Raftery (d.1835) from Killedan, near Kiltamagh (alias Kiltimagh) , who spent most of his life in south and east Galway, and whose numerous compositions included the ever-pupular 'Máire Ní Eidhin', 'Aithrí Reaftaraí' and, of course, 'Cill Liadáin'.
There were some stirrings in the west in the 1790s, with reports of agrarian disturbances in Tirawley, and an influx into Mayo of Catholic refugees from Ulster following the sectarian clashes in north Armagh in 1795 which led to the formation of the Orange Society. Nevertheless, when the United Irishmen were forced by government repression to move from working openly for reform to secretly plotting revolution, and when Leinster and east Ulster blazed into rebellion in June of 1798, no one expected Mayo to play a memorable role in the bloody drama about to commence. The man who dragged Mayo onto the stage of Irish history in 1798 was a French general from Lorraine, a former dealer in goat and rabbit skins named Joseph Amable Humbert.
Ten weeks after the United Irishmen had been crushed at Ballynahinch, Co. Down, and two months after the fall of the rebel camp at Vinegar Hill, near Enniscorthy in Co. Wexford, Humbert landed at Kilcummin strand, on Killala bay, with about 1,100 officers and men of the army of the French Republic. Four days later, on Sunday, 26 August, having taken Killala and Ballina, Humbert led about 700 of his men, and about the same number of untrained Irish recruits, in an amazing all-night march down the almost trackless west shore of Lough Conn, arriving next morning in front of the startled British garrison of Castlebar. The force opposing Humbert numbered about 1,700, under the command of General Lake, and consisted mainly of Irish militia. After a short, sharp engagement, the militia broke and fled, and were quickly joined by the remainder of the garrison in a headlong flight which, for some of them, did not end till they reached the safety of Tuam, Co. Galway. The episode, still remembered as 'the races of Castlebar', was an ignominious defeat for the government forces and a corresponding morale-booster for the small force opposing them, but it was in no way decisive. Humbert realised that without additional aid from France his expedition was doomed to failure. He remained in Castlebar for eight days awaiting further orders from his superiors, and while he waited he established a 'Republic of Connacht', with a young Catholic gentleman, John Moore from Moorehall on the shores of Lough Carra, as its president. When neither orders nor help were forthcoming, Humbert marched his little army towards Sligo, winning a skirmish at Collooney. Then hearing reports of a rising in the midlands, he swung south-eastward through Leitrim into Longford where, on September 8 the force of 850 French troops and about a thousand Irish allies faced a force over five times as strong under Lord Cornwallis and General Lake.
The token battle at Ballinmuck ended with Humbert's surrender after barely half an hour. The French soldiers were treated honourably, but for the Irish the surrender meant slaughter. There was more slaughter a fortnight later when Killala finally fell to General Trench's forces. The little garrison (including its commander, Ferdy O'Donnell) was massacred. The government forces were turned loose on the countryside. The insurgents, or anyone suspected of having been involved in the rising, were hunted down and butchered without mercy. In all, it is estimated that some four to six hundred were killed in the battle for Killala and in the course of the 'mopping-up operations' which continued for some weeks, while others died on the scaffold in towns like Castlebar and Claremorris, where the high sheriff for County Mayo, the Honourable Denis Browne, M.P., brother of Lord Altamont, wreaked a terrible vengeance - thus earning for himself the nickname which has survived in folk-memory to the present day, 'Donnchadh an Rópa' (Denis of the Rope). The awful aftermath of those few stirring weeks, in what was long remembered with a mixture of pride and horror, as Bliain na bhFrancach ('The year of the French') ensured that it was many a long year before the people of Mayo felt free to celebrate in song the exploits of "The men of the West' and to remind their countrymen that 'When Éire lay broken at Wexford she looked for revenge to the West.'