The early branch of gods is rarely discussed, not in academia nor in the myths, but understanding them is likely important to a deeper understanding of the various gods that descend from them.
Neit is the primary ancestor of both branches of the gods. He is in myth openly called the god of war. This does not mean he is akin to Mars however, though he shares characteristics. He is variously said to be married to Nemain, a goddess of battle terror, Like Lug he is also known from Celt-Iberian sources where Greeks identified him as both Mars and Apollo. From a philosophical perspective it is interesting also that a god of war is a progenitor, which seems to relate to a concept of continual cosmic strife, akin to Heraclitus' theory of the universe composed of fire and existing in a state of war. He is probably the embodiment of cosmic strife.
From Neit decend two different figures that go on to establish two separate branches of gods. One is headed by Delbaeth (likely the same as Tuireann Biccreo aka Taranis) who seem to be darker and more fierce. Many of them are associated with or are identified as Fomorians, chaotic and deadly gods to be appeased or fought rather than worshiped. Delbaeth is a great progenitor god but he is something of a dark and dangerous figure. The Three Gods of Danu, his sons, are called "the three gods known over every land for plundering". This isn't entirely a joyful description and they are depicted as being a-moral killers who will not spare Cian even when he begs for his life. They then lie to Lug about their guilt, another grave violation in ancient Ireland.
The second branch of gods originates with Esarg/Tuirbe Tragmar.There is a possibility that Neit is also the progenitor of another separate clan, the fomorians, but it may be that the fomorians are not so much a separate clan. Consider that Balor is described as grandson of Neit, identical to Bres, who is often stated to be the husband of the goddesses of the land. It is even possible that Balor is in fact another name for Delbaeth if some of the genealogies are to be believed, as sometimes Delbaeth is the father of Ethliu, mother of Lug. Any underworld, dark or sea deity is essentially a fomorian, therefore they likely do not constitute a separate identity as such but represent gods who are potentially harmful to human society and need to be controlled by the gods of order.
Delbaeth means something akin to "creative fire" or "form fire" and is likely identical to "delbaid" which refers to creation by magic with a negative connotation that implies deception, trickery or falseness. He is father of Elatha (skill/deception), who in turn is father to gods like Dagda, Ogma (maybe) Allot, Bres etc.
Esarg means "Smiting, striking, beating" but though taken initially as related to combat, and it is also meant that way, it is also simply the act of motor skills of the hand, using tools, cutting wood, forging a sword, etc. Thus, his sons are Goibniu, Dian Cecht/Cainte, Luchtra, Credna. If Delbaeth then is the power of physically manifesting elements, Esarg is father of gods who design things, who are greatly skilled in magic and arts and control the energy of creation to form and shape and heal.
It would seem however that Tuirbe Tragmar means "one who hinders the great tread/step". The only myth we know referring to Tuirbe Tragmar is that at high-tide he casts his axe upon the beach, and where his axe lands, the water shall not pass. This may well be the hindering of the great tread, the tread of the sea upon the earth that would overwhelm and drown it. He thus enforces the boundaries of land and sea, the limits between this world and the otherworld which is represented by the boundary of land and sea. What this shows is that, contrary to the view of many pagans today, the god Esarg, and the gods of his line, including Lugh Lamhfhada, are not primarily gods of physical material things, but they are gods who control and shape and limit and enforce. Esarg here shows the first and primary act of enforcement, the separation of the waters from the lands. He is not an element, not the sky, not the earth nor the wind or storm, he is a principle of delineation and law.
All Esarg's line is associated with channeling power to form, heal or to violently enforce cosmic law and oath (Lug). They are associated with various powers in the world, but are not reducible to them.