The Greeks used the term "Keltoi", meaning "Hidden People", to describe a number of different tribes who shared similar languages, cultures and customs, but were neither a homogenized group nor a unified nation. Some of the Keltoi were nomadic hunters and fierce warriors while others were settled farmers and consummate artisans. But, whether they were far wanderers or sedentary souls, this loose association of tribes maintained a recognizable cohesiveness of thought and attitude that persists to the present day among the people of the European Atlantic Façade. This is the Celtic sense of place, what the philosopher, Max Mueller, identifies as "an intimate relationship between language, spirituality and nationality".
Because Celtic tribal structure was built around place, an individual's position in society based on lineage, the genealogies of the Celtic people of the British Isles are some of the oldest in the world. Their complete social organization was dependant on the accuracy of the historical data bequeathed by prior generations and supported an entire stratum of society that faithfully maintained and passed this information down to their own descendants.
During the beginning centuries of the current era, political and religious motivations changed, molded by the world-view of the Roman Empire. The class of society charged with keeping the genealogies and the histories, the Bards, was essentially outlawed. Records that had never before been written down were recorded and altered to suit the current political climate. Spurious and extraneous data was added to create a common ancestor and promote the spirit of nationalism. All roads, in this case, led to Mil.
While it is not possible to detect all the bogus material, research in history, language, genealogy and archeology does allow us to apply logic and reason to a body of work that interweaves historic fact, linguistic constructs and verifiable genealogical data with numinous archetypes and outright fabrications. Surprisingly, if one discounts the application of specific information to individuals of dubious existence, the accuracy of the records is astonishing.
Most scholars propose 4 major Celtic incursions of Ireland:
| Cruithin | Picts | 8th-5th century BCE |
| Erainn | Firbolgs | 5th- 3rd century BCE |
| Laigin | Tuatha Dé Danann | 3rd-2nd century BCE |
| Gael | Milesians | 1st century BCE - 1st century CE |
Some researchers may question the dates, the extent of the incursions, whether the Goidel or Gael are actually a combination of the Connachta plus other tribes attached using fictional genealogies, and whether certain tribes fit into this group or that group based on conflicting traditions, but this is the generally accepted model. The Book of Glass is divided into sections based on this model: The Laigin of Lochlinn, the House of Heremon, The Picts of Pretani, the Children of Coel - and the Dragons of Dalriada, the amalgamation of all these races.
