The first appearance of Sinclair families in Ireland was a direct consequence of plantations promoted by James I during the early seventeenth century. Although individual Scottish Sinclairs may have visited during the reign of Elizabeth I, few seem to have settled.
Some Irish Sinclairs can trace their arrival from Scotland to the Ulster Plantation in 1610. These were probably younger sons taking advantage of the land escheated (taken back by the crown) from the Irish Earls who had fled after their defeat by the English. Many English and Scottish protestants settled in Ulster at that time. Life was hard, but they brought with them new farming skills and quickly increased the productivity of their lands.
One of the last plantations begun by James I was in King's County, now part of Co. Offaly in the Irish Midlands. This later plantation was not so successful as the Ulster Plantation, as noted by a Commission in 1622. Ely O'Carroll country was planted in 1619, and one of the principal undertakers was William Sinclair of Roslin. He and his wife, Jean Dobie, and their young family moved to Ireland and in 1624 William was naturalised (became a 'denizen').
Not all the early descendants of the Sinclair families established in Ulster and elsewhere during the seventeenth and eighteenth centuries became farmers. Some turned to the church, others to commerce and government, and some joined the protestant Irish who settled in North America when times were too bleak for them in Ireland.
Today, Sinclairs can still be found in Northern Ireland, but few know when their ancestors first arrived - other than sharing a common belief that they originated in Scotland. Genealogies of some of these Sinclair families are listed at other Irish Sinclair families.