Somewhere in Northern Ireland or possibly Scotland, prior to the end of the 1600's, there was a not terribly well off but hard-working family named Abernethy. We believe they were Protestants. Shortly after 1700, one member of that family left home and emigrated to colonial America, where he married, left descendants, and died in Frederick County VA in 1780. Another hard-working son of the family, born shortly before 1800, later moved across the Irish sea to Scotland, married and had a son who married in Dumfriesshire, Scotland in 1850; 20 years and several children later that son decided to move his family to New Zealand. A fourth son of that original family, born in Ireland about 1814, emigrated to Canada by 1841, married, was a blacksmith, reared a family, moved to the States, and died in 1896 in California.
A little over a hundred years later several descendants in different areas of the world began seaking their roots but hitting brick walls. They decide to have their Y-DNA tested to see if this new technological knowledge can help them. And it does: sketchy as it is at present, the story of that family is nevertheless unfolding, thanks to DNA data, which demonstrate that the four men from different areas of the world are, in fact, close kin, and probably descend from the same common ancestor who probably died no later than the 15th century.
Unfortunately, paper documentation from Ireland at that time is scarce, so positive proof such as we are accustomed to today may never be possible.
Although the process of establishing a DNA profile of a family is long and costly, there are some who believe it to be a worth-while endeavor.