St. Patrick, in his account of that famous vision by which he was called back to Ireland (Confessio 23), mentions people qui erant iuxta siluam Focluti quae est prope mare occidentale. By Tírechán and the author of the Tripartite Life this locality has been identified with the ‘ wood of Fochloth (Fochlad) ’ in Connacht, a place-name commonly believed to survive in modern Foghill (Fochoill) near Killala, Tirawley, co. Mayo. This view has been taken for an established tradition until recent times. Newport White and Bury were the first to find difficulties in reconciling the silua Focluti of St. Patrick's vision with the tradition placing his Irish captivity near Slemish in the northeast. Many attempts to solve the problem have since been made.

It is seldom possible to date accurately the beginning or end of population movements. It is doubtful whether the migration of Irish harvesters to Great Britain was very extensive before the end of the eighteenth century. At the close of the century, however, conditions were such as to render the migration desirable to the English farmers, and a necessity to the Irish peasant. By 1827 so desperate was the situation of the latter that a witness could declare before a parliamentary committee on emigration, that ‘ every decrease in the number coming to England each year would add to the probability of starvation in Ireland ’. The year 1838 has been taken as the closing date of this study since it saw the introduction of the first remedial effort (the Irish poor law act).

For the purpose of the extensive transfer of land to which the commonwealth government was committed, Benjamin Worsley, shortly after the date of his appointment in 1652 as surveyor general in Ireland, began to make a rough estimate of the land available. The slowness and inaccuracy of this survey caused a clamour among the soldiers and the adventurers. Large arrears of pay were due to the former, while many of the latter had, as early as 1642, advanced considerable sums of money for the purposes of the war in Ireland. Both classes wanted their estates with as little delay as possible. William Petty, then recently appointed as physician general to the forces in Ireland, became the severest critic of Worsley's proceedings. The upshot of the agitation was that the plan of survey was modified and Petty appointed on 11 December 1654 to map the forfeited lands set apart for the soldiers in 22 counties.