In "The Prophecy of Merlin" by Geoffrey of Monmouth, there are interesting references to astrology and the old gods. Some of them provide too little to go on, but for Mercury, Mars and Apollo at least there is some critical information. Here is the applicable section: "From them shall the stars turn away their faces, and confound their usual course. Corn will wither at their malign aspects; and there shall fall no dew from heaven. The roots and branches will change their places, and the novelty of the thing shall pass for a miracle. The brightness of the sun shall fade at the amber of Mercury, and horror shall seize the beholders. Silbon of Arcadia shall change his shield; the helmet of Mars shall call Venus. The helmet of Mars shall make a shadow; and the rage of Mercury pass his bounds. Iron Orion shall unsheath his sword: the marine Phoebus shall torment the clouds; Jupiter shall go out of his lawful paths; and Venus forsake her stated lines. The malignity of the star Saturn shall fall down in rain, and slay mankind with a crooked sickle. The twelve houses of the star shall lament the irregular excursions of their guests; and Gemini omit their usual embraces, and call the urn to the fountains. The scales of Libra shall hand obliquely, till Aries puts his crooked horns under them. The tail of Scorpio shall produce lightning, and Cancer quarrel with the Sun. Virgo shall mount upon the back of Sagittarius, and darken her virgin flowers. The chariot of the moon shall disorder the zodiac, and the Pleiades break forth into weeping. No offices of Janus shall hereafter return, but his gate being shut shall lie hid in the chinks of Ariadne. The seas shall rise up in the twinkling of an eye, and the dust of the ancients shall be restored. The winds shall fight together with a dreadful blast, and their sound shall reach the stars."
This is in reference to the end-times. We should note that Mercury is mentioned first of all, and is given command over the sun and light, and in this instance at least, dimming the sun. This is certainly in keeping with our established position in regards to Lug being a solar controller especially associated with the sunset and sunrise, a light bringer but also a light taker. It is likely in this dualistic aspect that he was thought to be a twin. It likewise associates Mercury with amber, something generally ascribed to Apollo. It cannot reasonably be a description of the color of the star, for Mercury does not appear amber coloured in the sky at any point. That this event should induce horror seems to imply that this is some kind of epiphany, perhaps a meteorite shower or comet appearing in the sky. It may be this which is called the amber of Mercury.
To realize the significance of this we need to return to our Greek sources. Apollonius had this to say in the Argonautica: "But the Celts have attached this story to them (amber), that these are the tears of Leto's son, Apollo, that are borne along by the eddies, the countless tears that he shed aforetime when he came to the sacred race of the Hyperboreans and left shining heaven at the chiding of his father, being in wrath concerning his son whom divine Coronis bare in bright Lacereia at the mouth of Amyrus."
Thus, despite being identified here as Mercury, the myth which is ascribed to this planet/god is that which the Greeks identified as the Celtic Apollo. But Merlin continues, saying that "Silbon of Arcadia shall change his shield". Silbon of Arcadia can be none other than Hermes, yet certainly no Hermes ever known to the ancient Greeks, who to my knowledge is never called this. Yet he is the god of Arcadia, along with Pan, who probably has the same origin in the Vedic god Pusan. What does Silbon mean? It seems to come from Latin Silbatus or Sibilo *to hiss. This is related to the receiving of prophecy. Think of Sybil, of the same etymological origin. Sibyli are female seers that made prophecy through a frenzied state. This seems to be an attempt at a masculine version of the name. Indeed this is known in modern Spanish, where it refers to whistling. Identifying this Silbon as being of Arcadia, directly after speaking of Mercury, identifies that they are simply using a poetic local term for Mercury as a prophetic god. Hermes was said to be born in the mountains of Arcadia, as was Pan. Neither, however, have any association with shields. However, there is a local god who does - Lug.
The earliest source we have of Lug is a praise poem called "The Shield of Lug". It is thought to be from the 7th century, likely written by a pagan, dedicated to a real or mythical king called Labraid, which seems to refer to this king as being "the shield of Lug". This position seems to be the most prestigious position one can have, for it goes on to say of him that he is "higher than the gods". This poem associates Lug with shields, and specifically someone or some god could hold the position as his shield, and perhaps this prophecy speaks of how it is a cataclysmic event when that person or god is cast out, or changed. The quick takeaway is that it is linking Mercury to being a prophetic god but also aligned with emblems of war like the shield which otherwise has no known association with the classical god Mercury or Hermes. This can only be explained by the local British knowledge and beliefs about the god they associated with Roman Mercury carrying on into the Medieval period. We will see a futher example of this momentarily.
The prophecy moves on to Mars, mentioning his helmet calling out to Venus and casting a shadow. Presumably it is casting a shadow because it has grown so bright. Unfortunately the only helpful information is that the focus is on the helmet rather than a spear. Also, in Roman Britain, the god identified as Mercury sometimes appears with the helmet of mars, or is sometimes even called Mars or Mars Silvanus, and so it unfortunately doesn't help us put an identity to this god.
The prophecy then returns to Mercury, highlighting the super-importance of this god in this end-times scenario. When compared to all other planets/gods mentioned, it is clear Mercury is the focus of the author's attention. He says "And the rage of Mercury pass his bounds". This can be taken in several different ways. The first thing to note however is that rage is not a trait ever associated with classical Mercury, Hermes or even their modern astrological interpretations. I have noted repeatedly the link between Lug and battle rage and transformative mental states and won't repeat it here, but this confirmation that indeed the god being discussed and identified by "Merlin" (Geoffrey of Monmouth) is Lug, or Lleu/Gwydion.
The Marine Apollo is a very interesting reference. This would seem to position Apollo as a potentially underworld figure, which perhaps matches what we know of Mabon, who is imprisoned, likely beneath the earth or the waves. That he assails the clouds is perhaps linking this to a struggle between chthonic forces and the sky deity, though it is pure speculation. Note however that in Irish myth there is conflict between Lug and the family of the Dagda, whose son is Mac Og, often assumed to be cognate with Apollo.
While the references to Mercury align with what we know of Lug from Irish and Welsh myth, unfortunately they do not help as much with identifying other gods. It also seems to show that despite the god having traits we might associate with Apollo, such as prophecy and an association with the sun, he was nevertheless often identified as Mercury, and archeological evidence from sites like Uley suggest he was likewise identified as Mars, Silvanus, or Mars-Silvanus.