Many of the more common family names, like MacDonald, Stewart, and Campbell occur in both places, and the 50 most common Scottish surnames can all be found in Ireland in one form or another. This is both a cause and effect of centuries of shared culture, history, and language between the Scottish and Irish people.
Evidence also points to there being, at one time, a land bridge between the two countries.
Throughout history there has been constant migration between Scotland and Ireland, made possible because of the relatively short distance between their coasts. Migration occurred for a variety of reasons, from deposed rulers and loyalists fleeing their enemies, to those who were simply settlers. In later centuries, much of the migration to Ireland was a result of English influence. During the era of Elizabeth I, many attempts were made to make Ireland more amenable to English rule, with limited success. To this end, the English set up plantations of Welsh and English in Ireland.