"I thought there were also clearances in Scotland, by Sutherland, the crofters were sent to live off the sea because their land was desirable, and some of them wound up going to Ulster, not to aid in settling, but to escape the horrible changes in their circumstances. Do I have events mixed up?"

Scotland had long been an exporter of people and people had moved back and forth over the short sea crossing between Scotland and Ireland for long enough. Hence there was considerable Scottish presence in the north of Ireland prior to the Plantation of Ulster. The difference with the Plantation was previously many Scottish incomers would be Highlanders or Islanders who were at that point culturally similar to the native Irish. The Plantation was a deliberate attempt to settle Ulster with culturally different people. The Scots who went, or were sent, tended to be Presbyterians, and mostly Lowlanders especially from south-west Scotland. The Plantation initially got started in about 1610. There was another big wave from the north-eastern Lowlands during the famine years of the 1690s. The Plantation as such was a phenomena of the 17thC especially the early years of that century. The Sutherland Clearances happened in about 1815 to 1820 so it was almost a full two centuries after the initial Plantation of Ulster and mroe than a century after the last great wave. So you are right in that not everyone who ever went to Ireland either to live permanently or on route to North America were actually part of the Plantation.