Looking at the 1901 Scottish census, there were 1633 people named McNair, scattered across Scotland. They won’t all be related.
As has been suggested, you should give us the names and approximate years of birth of ancestors you know were born in Scotland. Others may then be able to help you build on that. The more information you can provide the better, eg did they marry in Scotland and have children there (if so give details), their occupations in Scotland.
