Why are Conall Cearnach and Cu Chullain mentioned so much in the Fir bolg section of volume 4 of Robert Macalister's Lebor Gabála Érenn?

For example, on page 67, verse 8:

Cet Mac Magic from Mag Main,
Ros Mac Dedaid from Druim Cain,
Conall Cranach-- a solidity skinned over
Cu Chulaind, lord of a bag of tricks.

And they are mentioned many other times besides that... I'm trying to make sense of the context, but can't seem to. I mean, these two are heroes of the Red branch from the Ulster cycle which doesn't happen for a long time after the events of the rest of the section..

Am I missing something? Thanks

There is some overlap between the four cycles in Irish mythology. Particularly between the Mythological cycle and the Ulster cycle, being apparently the two earliest. In the tripartite birth story of Cú Chulainn (Compert Con Culainn), for example, the boy Sétanta is conceived three times. The first time he is born to a mysterious couple, who turn out to be Lug, of the Túatha Dé Dannan, and his wife, but when they disappear he ends up being raised instead by Deichtine, the daughter of the king of the Ulstermen, Conchobar mac Nessa. The second time, Deichtine falls asleep having unknowingly swallowed a small creature (like a flea or a worm) out of her cup before going to bed. She dreams that she sees Lug, and he tells her that she is now pregnant with his child, and that he should be named Sétanta. The third time she conceives with her husband, Sualtam mac Róich, but there is some implication in the story that the child is the same one all three times, and he is again named Sétanta. Hence, Cú Chulainn of the Ulster cycle is the child of Lug, of the Mythological cycle.

Turning to the specific poem you reference, it describes the wonderings of the sons of Umor, of the Fir Bolg (see pages 26-27). The poem initially refers not to Conall Cernach, but to a different Conall, the son of Aonghus (Oengus in the poem) of the Fir Bolg. This Aonghus, in turn, was one of the sons of Umor. The poem is set during the reign of Cairbre Nia Fer (in the poem, Coirpre), as king of Tara, which by the chronology presented in the Lebor Gabála Érenn, makes the events of the poem contemporaneous with those of the Ulster cycle. Thus, we see that Queen Medb and her husband, Ailill, give the land of Aidne to Oengus and his son, Conall in the poem.

The reference to Cú Chulainn and Conall Cernach in the poem, in verse 8, is where they are listed as two of the four sureties agreed between the sons of Umor and Cairbre. Along with them, Conall Cernach's maternal uncle, Cet mac Mágach, is also listed as a surety, as is Ros(s) Mac Dedaid. The point of these sureties is that they could enforce the contract made between Cairbre and the sons of Umor if they attempted to break the agreement. This is exactly what happens in the poem, and Cairbre summons the sureties in verse 16.