I have pieced the following together from talking with others; I've no documentation for any of it. The Shields clan was originally Saxon, living in England. After the Norman invasion, the Saxons, including the Shieldses, were forced into Scotland. There, they lived in the lowlands, became protestant, and generally squabbled with the Catholic and Celtic Highland clans.
James I of England (also known as James VI of Scotland) was having trouble with Catholics rebelling in Ireland in the 1600s. He decided to solve two problems at one stroke: He sent the protestant lowland clans, including such families as the Anthonys, the Hamiltons, the Kennedeys, and apparently the Shieldses, from Scotland to Ulster. Once in Ulster, the protestant clans made trouble for the Irish Catholics instead of for the English king. This is apparently how our ancestors ended up in Ulster, at least until they emigrated to the New World at various times in the 1600s and 1700s.
The Shields Family History, written by Benjamin Franklin Shields and published by him and the Shields Family in 1945, is now on-line. [Click Here]
Links to other web pages and sites for the Family