Merriam-Webster defines race as: any one of the groups that humans are often divided into based on physical traits regarded as common among people of shared ancestry.
(source: Merriam-Webster - RACE)

Alternatively, according to wikipedia, ‘Race’ is a categorization of humans based on shared physical or social qualities into groups generally viewed as distinct within a given society. The term came into common usage during the 16th century, when it was used to refer to groups of various kinds, including those characterized by close kinship relations. 

Side Note: Advances in the field of genetics in the late 20th century determined no biological basis for races in this sense of the word, as all humans alive today share 99.99% of their genetic material. For this reason, the concept of distinct human races today has little scientific standing, and is instead understood as primarily a sociological designation, identifying a group sharing some outward physical characteristics and some commonalities of culture and history.  

People whose ancestors have been living in the same geographic area for a long time tend to show similarities in visible characteristics such as size and shape, skin color, and hair form, and also invisible characteristics such as blood groups. Some of these groups are large, as were native Americans before Europeans and Africans came to the New World. Some are small, as are neo-Hawaiians (the descendants of Europeans, Japanese, and Polynesians). Large groups can be subdivided, depending on the level of comparison being made: sub-Saharan Africans are more similar to one another than they are to Europeans
(source: The National Center for Science Education | The Origin of Races)

Human "racial" diversity is a result of people in a geographic area intermarrying, being exposed to a number of biological processes, and adapting slowly to local environments. These biological processes include combining and recombining inherited genetic material over the generations, which produces offspring and descendants who differ from their parents and ancestors. The environment may favor certain characteristics, producing populations that are on the average taller, or darker, or more rugged than other populations from other geographic areas. Isolation and inbreeding of some populations may produce differences as well. These natural processes occur in humans as well as other animals and are the source of much study in biology and anthropology. More on the environment and weather with how it pertains to this specific example (redheads) later.
 

The Celts were a group of tribal societies and peoples who inhabited much of Europe in the Iron Age and the centuries leading up to the Roman conquests. They were known for their distinctive language, art, and culture. The Celtic peoples were spread across a wide area, including modern-day Ireland, Scotland, Wales, England, Germany, France, and parts of Central Europe.

The Celts were described by ancient Greek, and Roman historians as having distinct physical features such as Red hair, blue eyes, long limbs, and pale skin.

The following are direct quotes from the writings of some of those historians.

“For my own part, I agree with those who think that the tribes of Germany are free from all taint of inter-marriages with foreign nations, and that they appear as a distinct, unmixed race, like none but themselves. Hence, too, the same physical peculiarities throughout so vast a population. All have fierce blue eyes, red hair, huge frames, fit only for a sudden exertion. They are less able to bear laborious work. Heat and thirst they cannot in the least endure; to cold and hunger their climate and their soil inure them.”

• Tacitus ( source: Complete Works of Tacitus )

Later Tacitus in his Agricola, chapter XI (c. 98 AD) described the Caledonians (what the Romans’ called the inhabitants of modern day Scotland) as red haired and large limbed, which he considered features of Germanic origin: "The reddish (rutilae) hair and large limbs of the Caledonians proclaim a German origin".

“...The inhabitants of Caledonia have reddish hair and large loose-jointed bodies.”

• Jordanes in his Getica

Eumenius, the panegyrist of Constantine Chlorus, wrote that both the Picts and Caledonians were red haired (rutilantia). Scholars such as William Forbes Skene noted that this description matches Tacitus' description of the Caledonians as red haired in his Agricola.[21]

( source: Caledonian Physical Description )

4th-century Roman historian Ammianus Marcellinus wrote that the Gauls were tall, light-skinned, light-haired, and light-eyed:

"Almost all Gauls are tall and fair-skinned, with reddish hair. Their savage eyes make them fearful objects; they are eager to quarrel and excessively truculent. When, in the course of a dispute, any of them calls in his wife, a creature with gleaming eyes much stronger than her husband, they are more than a match for a whole group of foreigners; especially when the woman, with swollen neck and gnashing teeth, swings her great white arms and begins to deliver a rain of punches mixed with kicks, like missiles launched by the twisted strings of a catapult."

( source: Gauls | Physical Appearance | Wikipedia )

If you recall the explanation given to the second question "How are 'races' formed?" one of the main factors was determined by experts to be the Weather.

Scottish research has found that the Celts' flaming red hair can be put down to the weather. With ten percent in Ireland and 13 percent of people in Scotland boasting glorious red hair. The experts believe that the gloomy climate in Scotland prompted a deliberate genetic adaptation. Essentially, this means that red hair helps to take advantage of sunny days and allows the body to absorb more vitamin D.
(source: Irish Central)

So you might be wondering to yourself, “where are all the redheads then? If these lands were once populated entirely by Celts who were described as all being red haired… how come there are so few remaining in that region today?”

The following point will give some explanation as to why.

As the burgeoning Roman Empire began it’s expansion into North-Western Europe, it’s armies encountered the Celtic tribes. Over time the Roman people intermarried, and interbred with those indigenous inhabitants. They also fought Celtic tribes and enslaved the Redhaired Celts.. Roman slaves with red hair sold at a higher price, and It’s been documented that wigs of red hair were imported from northern Europe, for slaves to wear to increase their value. 

This tumultuous period of time caused the population percentage of indigenous Celts to drop dramatically.

Looking at the several images we've provided below (click them and you'll see a description for each), there is a clear correlation between the area of land where the Roman Empire ceased it’s expansion, and the modern day population percentage of red haired people in Europe.

 
It is also no coincidence that the same region is the home to the highest percentage of people who speak Celtic languages today. 

 Think that this couldn’t happen? Well it has happened… many times. You see, just as those fiery haired Celtic people were forced to fight for their ancestral lands in the face of encroaching armies such as the Romans, so too did indigenous populations across the Americas, Australia, and Canada face the relentless migrations and colonization of European settlers.

Estimating the pre-Columbian population of Native Americans is challenging due to limited records. But, historians and anthropologists estimate it was likely in the tens of millions, with some suggesting numbers ranging from 10 million to over 100 million across the Americas.

As far as more recent estimates go, the current population of Native Americans in the United States is approximately 5.2 million, while in Canada, it's around 1.7 million…

Meanwhile, there are approximately 197 million Caucasian or white people in the United States.

The point is, it is possible for a dramatic population “flip” to occur in very little time in regions much larger than that of the United Kingdom and Ireland.

The parallels between the plight of the ancient Celtic people and the experiences of indigenous populations elsewhere around the world serve as poignant reminders of the enduring struggles for autonomy, dignity, and cultural survival in the face of colonial oppression.

Despite the passage of time, the echoes of displacement and dispossession continue to reverberate through history, underscoring the importance of recognizing and honoring the resilience of those who resisted, and endured the harsh realities of conquest and colonization.