Stewart of Atholl is a cadet of the Royal Stewart [or original] line. It is an offshoot of the original family. Technically, the Stewarts are not of "clans" but rather of Noble Families. Today, they are commonly referred to as "clans."
"The Stewarts of Atholl descend from a son of the Wolf of Badenoch. James Stewart built a strong castle at Garth and settled there towards the end of the fourteenth century. In 1437 Queen Joanna, widow of James I, married the Black Knight of Lorne who was descended from the fourth High Steward. Her son by this marriage, Sir John Stewart of balveny, was granted the earldom of Atholl by his half-brother James II. He supported his brother by commanding the royal forces who suppressed the rebellion of the Lord of the Isles. John, fifth Earl, did with no male issue. His daughter had married William Murray, second Earl of Tullibardine, woh was created Earl of Atholl in his own right in 1627. Many Stewart families continue to live around the Atholl lands, many of them claiming direct descent from the numerous progeny of the Wolf of Badenoch. They largely transferred their alliegiance to the new Murray Earls of Atholl, calling themselves Athollmen. This is commemorated by the right still exercised by the present Duke of Atholl to maintain the Atholl Highlanders as the only private army in the Kingdom. The new allegiance was sometimes sorely tried. In 1689 Murray called out the Athollmen for William of Orange, but his baillie defiantly held Blair Castle for James II. In 1715 Atholl again supported the government but his heir, the Marquess of Tullibardine, was a Jacobite. Stewarts flocked to the banner of Bonnie Prince Charlie in 1745. General David Stewart of Garth, an Athollman, was an officer in the 42nd Regiment (the Black Watch) whose book, "Sketches of the Highlanders and HIghland Regiments, " did much to popularize his homeland to Victorian England." Quoted from p 460-61, "Scottish Clan and Family Encyclopedia," by George Way of Plean and Romilly Squire.
The organization that supports the activities of the Stewart clans in the USA and Canada is Clan Stewart Society in America. You will find the website at http://www.clansstewart.org
Note there are two "s" for clans Stewart.
CSSA attends many Scottish Highland Games and Gatherings, as well as Celtic Festivals. To see a comprehensive listing of such events in the USA, please visit the Scottish Information Society site http://www.scottishinfo.org
and the page devoted to the games and festivals http://www.scottishinfo.org/scottish.htm
and then click the "2006 Games Listing" link. You will not find a Stewart tent at every event in the country as it is just not possible to find volunteers to man them all. However, you can check the listing and then contact the Region Commissoner for the area and ask if they will have a tent at the events near you. http://www.clansstewart.org/region_information.htm
I hope this information is helpful to you.
Lauren M. Boyd, FSA Scot
Chair, Genealogical Committee
Clan Stewart Society in America