Most sources say the Dál Cuinn are descendants of Éremón son of Milesius. However, The Laud Genealogies and Tribal Histories give a much different origin. This is a very loose translation (the original link appears to be broken):
This source says the Dál Cuinn were a later invasion led by a SECOND Milesius. I have speculated elsewhere that this MAY have been Crimthann Nia Nár, a not too distant ancestor of Conn Cétchathach (which I am completely convinced is not a name, but a title meaning “Leader of 100 Battalions”). Further, I have also speculated that the myths of the Túatha Dé Danann are actually a distortion of the invasion of the proto Dál Cuinn.35. So that ye may know the origin of Dál C. and their exploits.
36. Three sons of Búan mac Lóegaire Birn, from whom are [descended] the Osraige; i.e. Ailill, Óengus [and] Fíacc, who first planted house-posts in Tara.
37. It is the time in which the second Míl Espáine in Ireland came to them. Three fifties of men their number.
38. He reached Delossath Beach and advanced inland and wreaked great devastation and captured land there by the sword. {Alternate -- He seized the shore by fury etc. }
39. Thus they took lordship from then on, and thus it will be always, (that it is) by force they take lordship.
40. His progeny is this: Dál Cuinn and Dál Fíachach. Some say that the Shining Éogan is [one] of the mariners that they left in the land of the Amazons, i.e. among the breast-cauterised ones.
41. The last Míl Espâne, however, it seems to us that he was of the people whom they left in (the) city of Breogan.
42. And it is after that [i.e. after they left the land of the Amazones] that they came to Ireland, with the clays of the city of Themis in their possession.
43. It is those clays which were thrown down in Tara after reaching Ireland, so that Tara is the name of that high place, i.e. theme [? = Latin thema 'that which is set down'?].
44. It is [now] the final era, of which it was said: the sovereignty will be Síl Cuinn's and Mumu's forever.
45. Cruithni contested the sovereignty with them. It is for that reason Fînán mac Fîachrach composed [the following] concerning the Dál nAraidi:
Provided he didn't fight for Tipraite
I raised an army for the latter.
Seven battles and forty
and seven hundred more.
46. Cenél Cuinn gave battle, they broke upon them in a ship. For sovereignty had belonged to the Cruithni, and the final invasion fell to it [i.e. to Cenél Cuinn] on that northern shore (northern side?).
47. However, sovereignty was conceded to them alternately until it came to him, namely Conn Cétchathach.
48. Indeed Conn Cétchathach fought a hundred battles and a hundred conflicts concerning that sovereignty against the Dál nAraidi.
This invasion appears to have been on the northwest coast of Ireland. I surmise that “Delossath Beach” is a corruption of Tracht dé Iorrath/Iorrus or “Beach of the Smokey Haze of Iorrath/Iorrus”. This is echoed in the tales of the Túatha Dé Danann wherein they burn their ships on the beach after arriving in Ireland or arrive in Ireland on clouds of mist and smoke - the first being my preferred interpretation.
Please see my map in The Original Teóra Connachta topic. Note that the territory of the Fir Domnann includes much of the northwest coast, particularly Umall or the territory of the descendants of Conall Oirbsen son of Brión. Oirbsen COULD mean “the old homestead”. Hubert Thomas Knox in his article entitled “The Early Tribes of Connaught - Part I” page 354 makes a mention of the connection between the Domnann and Danann.
So we have the possibility that the proto Dál Cuinn may have been originally called the Fir Domnann (from which came the later Túatha Dé Danann, but I am not convinced of the connection with the British Dumnonii) and spread out from Umall. And is it possible that Túatha Dé Danann originally meant the “People of the Smokey Haze of Domnann” and not “People of the goddess Danu”? That there was a later conflation?The Cruithne of Leinster, and the Túatha dé Danann, and the Firgaileoin appeared to be the same race. The Danonians are a branch from the Firgaileoin; as Firgaileoin are certainly Firbolg, and apparently Domnonian, the Danonians are also really Domnonian in origin.
Regardless, Brión seems to be most closely associated with Damchluain near Tuam. And the Uí Briúin Seóla capital was on Loch Cime near Headford, not far from Tuam. The Irish for Headford is Áth Cinn. However, it is easy to speculate that the name was originally Áth Cuinn or the “The Ford of Conn” and there was a later corruption; because áth usually means a ford that is commonly a scene of battle or single combat.
As mentioned elsewhere, all the evidence indicates that all 3 major branches of the Uí Briúin were originally located in Mag Seóla and spread out from there beginning in the mid 700s AD. With Tuam and Headford having associations with Brión, either directly or indirectly, it seems most likely this area was the original seat of Brión's territory; with Ráth Crúachán being reserved strictly for the current King of Connacht.
Given that Conall Oirbsen's territory was in Umall, and from there as an epicenter given the historical spread of the Dál Cuinn into Connacht and Ulster, it seems very likely that this is where the proto Dál Cuinn first arrived. The archaeological evidence of the Doon of Drumsna indicates they most likely came from the La Tène region of Continental Europe; or at the very least were quite familiar with construction techniques of fortifications from that region. I am not aware of any similar sites in the Dumnonii territory of Dumnonia (Cornwall and Devon).
This is based on a lot of speculation and pulling together a lot of information from disparate sources, but it does offer a reasonable explanation for the locations of the known Dál Cuinn territories and their spread into Ireland.
