Visions danced before her, gold and scarlet, flickering, forming and melting and dissolving into one another, shapes strange and terrifying and seductive. She saw the eyeless faces again, staring out at her from sockets weeping blood. Then the towers by the sea, crumbling as the dark tide came sweeping over them, rising from the depths. Shadows in the shape of skulls, skulls that turned to mist, bodies locked together in lust, writhing and rolling and clawing. Through curtains of fire great winged shadows wheeled against a hard blue sky.
The eyeless faces are revealed later to be rangers blinded and killed by the Weeper. The towers by the sea are claimed by Melisandre to be Eastwatch, but she notes that they didn't actually look like the same towers at P. 416, where she also describes the tide as "black and bloody". While she puts this down to the way prophecy works, it suggests to us that some quite different towers are intended. One possibility that springs to mind is that the towers might be the towers of Greenstone on Estermont, which we learn is taken by elements of the Golden Company, but that doesn't seem very likely. The rising from the depths might make one think of krakens and so the ironmen, and there happens to be a mainland castle called Three Towers which might be threatend by Euron Greyjoy.
The skulls appear to be a reference to death, although Melisandre does briefly consider (and then dismiss) the possibility that they refer to the Bridge of Skulls. The writhing, lustful bodies might possibly refer to glimpsing Daenerys with Daario. Although Melisandre has previously never had any sort of glimpse of Daenerys, so far as we know, the fact that it seems she glimpses dragons immediately following might be in support of this.
The girl. I must find the girl again, the grey girl on the dying horse. Jon Snow would expect that of her, and soon. It would not be enough to say the girl was fleeing. He would want more, he would want the when and where, and she did not have that for him. She had seen the girl only once.
A girl as grey as ash, and even as I watched she crumbled and blew away.
As discussed earlier, this is Alys Karstark, not Arya Stark as Melisandre believes.
A face took shape within the hearth. Stannis? she thought, for just a moment . . . but no, these were not his features. A wooden face, corpse white. Was this the enemy? A thousand red eyes floated in the rising flames. He sees me. Beside him, a boy with a wolfs face threw back his head and howled.
This is clearly a glimpse at Brynden Rivers, Bloodraven, the last greenseer and the three-eyed crow, and with him Bran Stark.
Snowflakes swirled from a dark sky and ashes rose to meet them, the grey and the white whirling around each other as flaming arrows arced above a wooden wall and dead things shambled silent through the cold, beneath a great grey cliff where fires burned inside a hundred caves. Then the wind rose and the white mist came sweeping in, impossibly cold, and one by one the fires went out. Afterward only the skulls remained.
Death, thought Melisandre. The skulls are death.
This appears to refer to the caverns at Hardhome, and the horde of wights coming to destroy the wildlings who followed Mother Mole there. The wooden wall may be some remnant of the defenses of Hardhome, or something erected by the refugees or rescuers there. It does seem like things shall go very poorly, at least at the time of that vision.
The flames crackled softly, and in their crackling she heard the whispered name Jon Snow. His long face floated before her, limned in tongues of red and orange, appearing and disappearing again, a shadow half- seen behind a fluttering curtain. Now he was a man, now a wolf, now a man again. But the skulls were here as well, the skulls were all around him.
The flickering between man and wolf may simply refer to Jon Snow being a warg... but given the events at the end of the novel, this may instead refer to his warging into Ghost at his death, and ultimately being placed back in a human form.
Skulls. A thousand skulls, and the bastard boy again. Jon Snow...
Yet now she could not even seem to find her king. I pray for a glimpse of Azor Ahai, and R’hllor shows me only Snow.
This appears to be key. Melisandre, absolutely sure that Stannis is Azor Ahai reborn, is ignoring the fact that again and again the flames are showing her Jon Snow when she asks to see Azor Ahai reborn. This appears to be strong evidence for the idea that Jon Snow could very well be the one to fulfill the prophecy that Melisandre has previously spoken of.