I finally found a reference to early Southerners and a particular yell associated with one of the most famous Battles fought in the South during the War of Independence, the Battle of King's Mountain.
[But before Shelby's men could gain their position, Colonel Campbell had thrown off his coat, and while leading his men to the attack, he exclaimed at the top of his voice, — "Here they are, my brave boys; shout like hell, and fight like devils!" The woods immediately resounded with the shouts of the line, in which they were heartily joined, first by Shelby's corps, and then instantly caught up by the others along the two wings.* When Captain De Peyster heard these almost deafening yells — the same in kind he too well remembered hearing from Shelby's men at Musgrove's Mill,— he remarked to Ferguson: "These things are ominous — these are the damned yelling boys!"** And when these terrific shouts saluted Ferguson's ears, he expressed fears for the result.***
About the time the Virginians advanced to the conflict, Major Micajah Lewis, with his brother, Captain Joel Lewis, both of the Wilkes and Surry troops, with Captain Andrew Colvill, of the Virginia regiment, had been designated by Colonel Campbell to make a dash on horseback upon the British main guard, half way up the spur of the mountain; and having swept them out of the way, to fall back, dismount, and join the others in the general advance. Here the first heavy firing took place between the contending parties, the guard commencing it. The mountaineers raised the Indian war-whoop and rushed upon the foe, who soon retreated, leaving some of their men to crimson the earth with their blood.****] King's Mountain and its heroes: history of the Battle of King's Mountain ...By Lyman Copeland Draper, Anthony Allaire, Isaac Shelby, 1881. Page 247.
*Statement of John Craig, one of Campbell's men; conversations with Gov. David Campbell, in 1844.
** Statement in 1844 of Col. George Wilson.
***Gov. Campbell's statement
****Statement of J. L. Gray, and his communication in the Rutherfor Enquirer, May 24th, 1859.
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Another reference . In 1780 in the Tennessee territory the pioneers were known for their fighting style against the local natives.
[But what strikes us with most wonder is that, in the midst of this human holocaust, there was light-hearted gayety throughout the settlements. These men seem to have been insensible to fear, and to have delighted in danger. With ready alacrity, and at desperate odds, they rushed into battle with the savages. Steady and undaunted in defense, they were impetuous and irresistible in attack. With the "Tennessee yell" they had learned of Sevier, {General John Sevier} they would swoop like a whirlwind upon the enemy, never asking or expecting quarter. It was always a life-and-death struggle. Never did one submit to be captured. Not a solitary case of cowardice is reported; but there are numberless instances of individual bravery, of disinterested friendship, and self-sacrificing devotion, that will compare with the most heroic achievements in history. In the most perilous crises there were always bold spirits ready to break the cordon of savage fire, to secure food for a hungry garrison.]
The advance-guard of western civilization, By James Roberts Gilmore, 1888, page 19.
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[The Cumberlander always yelled in battle; his war whoop was his drum, fife, uniform, and flag; it not only gave him heart but it sometimes tricked the Indian, for many yells betokened many men, and so they must have yelled that night, at least the men*] Seedtime on the Cumberland, page 291.
*The Indians raised the whoop the whites raised the yell….The trader James Adair had an unusually complete mastery of the Chickasaw whoops and could give many; his “American Indian” pg 295, “shrill whoop of friendship”, 296, “shrill whoop of defiance”; 347, the “death whoop-whoop.” Which of these fathered the Rebel Yell I do not know, possibly a combination.
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[This is precisely the Indian whoop. They raise a loud scream, at the same time clapping the mouth rapidly with the open hand. When the expiration is nearly finished, they cease the clapping with the hand, and collecting all their remaining force, suddenly raise the scream an octave higher, and then cease for the moment.] Researches, philosophical and antiquarian, concerning the Aboriginal History of America ...By James Haines McCulloh, 1829
[When the warriors are arrived within hearing, they set up different cries, which communicate to their friends a general history of the success of the expedition. The number of the death cries they give, declare how many of their own party are lost; the number of war whoops, the number of prisoners they have taken. It is difficult to describe these cries, but the best idea I can convey of them is, that the former consists of the sounds whoo, whoo, whoop, continued in a long shrill tone, nearly till the breath is exhausted, and then broken off with a sudden elevation of the voice. The latter is a loud cry, of much the same kind, which is modulated into notes by the hand being placed before the mouth. Both of them might be heard to a very considerable distance.] Very interesting this passage because it is found in "The journal of Lewis and Clarke to the mouth of the Columbia River beyond ...
By Meriwether Lewis, William Clarke, Harvard, 1840; but I've also found the exact passage it books written in 1778! "Of the Manner of making War amoungst the Indians of North America. From Carver's Travels."
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Another reference gives the origin of the "Tennessee Yell" directly to General John Sevier and there is good reason. I am convinced that the Tennessee and Rebel Yell are linked and it is most likely a copy of Chickasaw war whoops or Cherokee war screams...it's origin is not Celtic.
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David Upton