You can't find "the truth" about the Celts from any one source, including mine.

"Celtic identity is a chronically vague and indefinable concept, but also one of special importance at the present stage in our history. (It is) a subject of profound uncertainty and over-heated debate amongst both the public at large and academic specialists." (Source: The Celtic Languages and Cultural Identity: A Multidsiciplinary Synthesis)

Some Obstacles to Defining Celtic Culture

• geographical distribution
  Celtic lands in western and central Europe covered an area as large as the USSR before its breakup into smaller nations at the end of the 20th century.
• time distribution
  Even among scholars there are differences of opinion about when to date the origin (and final days) of Celtic culture.
• tribal organization versus national organization.
  There was no Celtic "empire." Despite their proliferation, the Celts were never a united nation; rather, they were members of numerous clans, loosely affiliated with independent tribes that had both shared and differing cultural components. Furthermore clan and tribal affiliations were changeable.
• biases
  • A male bias in archaeology and anthropology, two of the primary ways of knowing about the Celts, is being qualified and quantified by current scholars looking at gender in those disciplines.
  • The bias in the academy for Greek culture (art, literature, philosophy, science and mythology) as the foundation of "western" civilization has resulted in the neglect of the serious and thorough study and appreciation of other roots of western civilization, including the Celtic roots.
  • The Celts left no written record, other than some inscriptions on stone. The written sources are these: the Greeks, who traded with the Celts, the Romans, who conquered them, and the Christians, who converted them and transcribed some of their oral histories centuries after the death of Celtic culture, or at least after the death of pagan/barbarian/heathen Celticity. These stories were written by Catholic monks, so what we have is what has been passed on to us through three major biases: male (gender) bias, Roman (appropriation) bias, and Christian (religious) bias.
  • Romanticism: Much of what is popularly considered to be "Celtic" comes from the 19th century romantic movement and is based on fancy moreso than fact. This is also true for much of the information on Celtic culture and religion coming from the 20th century neopagan movement.
• ignorance
  We still have a very sketchy idea of "prehistoric" Eurasian migration patterns and populations.
• disagreement
  There is no single story of Celtic culture. For example, to understand Irish history and culture alone, a non-student learner was advised by an Irish history expert at a major US university to read six (specific) books on the subject to begin to get a picture of Irish history -- three scholarly, respected books from one perspective and three from an opposing perspective. The Irish and Ireland have an important place in Celtic culture, especially in terms of the relatively small impact the Romans had on them and relatively late impact of Christianity on their culture. Nevertheless, Irish Celtic culture is but part of a larger, exponentially varied culture.