In the Táin Bó Cúailnge, or Cattle Raid of Cooley, the great Irish hero Cuchulainn is said to undergo a process which appears to be quite similar to "hulking out". The most colorful translation of the phrase used to describe this transformation is "warp spasm".
Hulking out is a fairly appropriate comparison to the riastrad, or warp spasm, as Kinsella put it, but far more monstrous than just green-tinged and well-muscled.
Massively muscled, lower legs twisted around backward, one eye sucked into his head, the other eye having fallen outward, cheeks peeled away from his mouth to reveal his jaw, his own internal organs visible in his mouth, hair stood up in massive spikes strong enough to skewer an apple, and blood spurting from his forehead. Rather intimidating all told.
The fullest description occurs after the slaying of the boy troop of Emain Macha. Kinsella's translation puts it this way:
The first warp-spasm seized Cúchulainn, and made him into a monstrous thing, hideous and shapeless, unheard of. His shanks and his joints, every knuckle and angle and organ from head to foot, shook like a tree in the flood or a reed in the stream. His body made a furious twist inside his skin, so that his feet and shins switched to the rear and his heels and calves switched to the front... On his head the temple-sinews stretched to the nape of his neck, each mighty, immense, measureless knob as big as the head of a month-old child... he sucked one eye so deep into his head that a wild crane couldn't probe it onto his cheek out of the depths of his skull; the other eye fell out along his cheek. His mouth weirdly distorted: his cheek peeled back from his jaws until the gullet appeared, his lungs and his liver flapped in his mouth and throat, his lower jaw struck the upper a lion-killing blow, and fiery flakes large as a ram's fleece reached his mouth from his throat... The hair of his head twisted like the tangle of a red thornbush stuck in a gap; if a royal apple tree with all its kingly fruit were shaken above him, scarce an apple would reach the ground but each would be spiked on a bristle of his hair as it stood up on his scalp with rage.
Which is actually fairly concise, compared to the very direct translation of Joseph Dunn, which goes on for four paragraphs in full description of the process.
Then took place the first twisting-fit and rage of the royal hero Cuchulain, so that he made a terrible, many-shaped, wonderful, unheard of thing of himself. His flesh trembled about him like a pole against the torrent or like a bulrush against the stream, every member and every joint and every point and every knuckle of him from crown to ground. He made a mad whirling-feat of his body within his hide. His feet and his shins and his knees slid so that they came behind him. His heels and his calves and his hams shifted so that they passed to the front. The muscles of his calves moved so that they came to the front of his shins, so that each huge knot was the size of a soldier's balled fist. He stretched the sinews of his head so that they stood out on the nape of his neck, hill-like lumps, huge, incalculable, vast, immeasurable and as large as the head of a month-old child.
He next made a ruddy bowl of his face and his countenance. He gulped down one eye into his head so that it would be hard work if a wild crane succeeded in drawing it out on to the middle of his cheek from the rear of his skull. Its mate sprang forth till it came out on his cheek. His mouth was distorted monstrously. He drew the cheek from the jaw-bone so that the interior of his throat was to be seen. His lungs and his lights stood out so that they fluttered in his mouth and his gullet. He struck a mad lion's blow with the upper jaw on its fellow so that as large as a wether's fleece of a three year old was each red, fiery flake which his teeth forced into his mouth from his gullet.
There was heard the loud clap of his heart against his breast like the yelp of a howling bloodhound or like a lion going among bears. There were seen the torches of the Badb, and the rain clouds of poison, and the sparks of glowing-red fire, blazing and flashing in hazes and mists over his head with the seething of the truly wild wrath that rose up above him. His hair bristled all over his head like branches of a redthorn thrust into a gap in a great hedge. Had a king's apple-tree laden with royal fruit been shaken around him, scarce an apple of them all would have passed over him to the ground, but rather would an apple have stayed stuck on each single hair there, for the twisting of the anger which met it as it rose from his hair above him.
The Lon Laith ('Champion's Light') stood out of his forehead, so that it was as long and as thick as a warrior's whetstone. As high, as thick, as strong, as steady, as long as the sail-tree of some huge prime ship was the straight spout of dark blood which arose right on high from the very ridge-pole of his crown, so that a black fog of witchery was made thereof like to the smoke from a king's hostel what time the king comes to be ministered to at nightfall of a winter's day.
Just for fun, a number of comics have been made about and inspired by Cuchulain and the Tain. Here are a couple instances of a riastrad in comics:
Cover of Richard Knaak's "The Hound"
Was to be published in 2013, but I can't find it for sale
Patrick Brown's "The Cattle Raid of Cooley"
Chapter 7, Page 20
From the "Slaine" graphic novel
Inspired by the Tain and Cuchullain
Artist: Glenn Fabry
Another from "Slaine"
Which all seem at least a little on the conservative side to me, given the incredible, hyperbolic description in the text.
It has been equated by many to the Viking Berserkers who became like mad beasts, biting on their shield rims, howling madly and impervious to sounds even though they went into battle unarmed and even shirtless (i.e. Bare Shirts). As in both cases some sort of France effect, perhaps aided by drugs, was involved. It can't be overlooked at the base of the exaggerated descriptions were how the warriors themselves felt under the fate induced. Then, too, think of news descriptions of drug addicts impervious to police restraints and tasers. However, finally, part of the descriptions were shared by other ritual figures, such as druids similarly taking on weird postures and facial distortions when uttering curses or satires. Lastly, the spiked hair and other affects may have been part of the champion warriors expected appearance. Celtic warriors, for instance, often fought naked and spiked their hair with lime and did other cosmetic things like paint their bodies. When the formerly invincible hero Cuchulain is beaten in single combat his vanquished cuts off his golden locks. An act so humiliating some liken it to being raped.