Desmond's Concise History of
From Chapter 7: "Early 18th Century Ireland: The Plantation System"
the "Williamite Plantation"
Short term, the plantations were enormously successful for England. In 1603, 95% of land in Ireland was owned by Catholics; by 1701, less than a century later, only 14% was owned by Catholics, an aggregate transfer of 81% of all productive land in Ireland. Further, the percentage of non-Irish in the population had been increased from 5% to 25%. It is possible that the Crown expected the Irish and British cultures to merge eventually (with English culture predominating, naturally), but of course this did not happen. Instead, the Plantations divided Ireland, apartheid-like, into two hostile camps, a socio-economic tinder box virtually certain to explode.
-- In one camp was 75% of the populace: poverty stricken, landless, ethnically Irish, Gaelic speaking, Catholic, and powerless; these descendants of pre-17th century natives thought of themselves as Irish, not English, and were more hostile than ever before towards their English conquerors.
-- In the other camp was 25% of the populace: affluent landed gentry, ethnically English or Scots, English speaking, Protestant (Anglican [10%] and Presbyterian [15%]), and politically dominant; these immigrants thought of themselves as the Crown's colony in Ireland, not as Irishmen (although within a few generations they began to regard themselves as a "Protestant [Irish] nation").
Plantations were the medieval equivalent of "ethnic cleansing" in that -- in theory at least -- all occupants of confiscated land were to be evicted and resettled in Connacht where they would be less of a military threat. Anti-Catholic animus played a role in the plantations, but other motivations were more important. For the new English immigrants to Ireland, the principal motivation was obtaining fertile land at bargain rents. For the English monarch, plantations deprived dissident Irish lords of the land that had been their only real source of power. Further, the new loyal non-Irish minority served as an unpaid police force to keep dissident Irish Catholics in check.
Although directed at the aristocracy, the plantations devastated peasants who suffered the loss of their property rights under the ancient Gaelic law of gravelkind, which previously had guaranteed them a decent living from the soil. It turned out that peasants were needed for hard labor, so many of them, despite the original "resettlement in Connacht" plan, were allowed to remain as tenant-farmers, but at low wages with backbreaking rents that thrust them into abject poverty.
Predictably, both in resentful peasants and in their Gaelic lords, there developed a 300 year obsession -- sometimes violent, sometimes political -- to overturn or modify the confiscations via "land reform", a term which (depending on time and place) might mean anything from a complete reversal of the confiscations to a modest improvement in tenants' rights.