Arguably the most popular and influential character in Irish folk history, Finn mac Cumhail (approximately pronounced 'Fin Mac Coo-ull') is sometimes referred to informally as "The Hibernian Hercules" because of this central role. The stories of Finn and his band of warriors are collectively referred to by students of Irish history as The Fianna Cycle. In this write-up, I will try to trace a brief outline of the life of this great hero. First, however, a bit of preface and explanation.
The Gaelic language began as a solely oral (that is, spoken) language. The Irish of old did not employ the written word until Christian monks brought it with them in the sixth and seventh centuries. As a result, there are many different English spellings floating around for most older Gaelic words and names, Finn's being one of them. Finn is often spelled Fionn, and the spelling of his surname varies widely, including mac Cuhal, mac Cumhaill and everything in between. The modern anglicized version is "McCool", and this is not an uncommon Irish or Irish American surname. At any rate, all of these spellings are equally valid - I've chosen Finn mac Cumhail for this write-up simply because the node already existed.
It must also be kept in mind that, because most of the tales of Finn began before the written word was established in Ireland, they are a part of folk history rather than written history. Over the span of the many generations during which these stories were told and retold, they undoubtedly underwent changes. Perhaps they were made more colorful by one teller, or place names were changed by another to be closer to home, or the overall bias was changed due to current Clan politics. The result of all these factors is that there are many versions of each of Finn's tales, not all of which could be easily represented here. However, when filtering through the vast variety of stories, certain common threads and events emerge. It is these most widely accepted tales that I will briefly recount here.
To introduce Finn, first we must introduce his parents and the setting into which he was born. Ireland at this time was a loose collection of warring Clans, large extended families, with a complex and fragile network of alliances and feuds. There was an Ard Rig, or high king, who sat upon the throne at Tara and claimed rulership over the island, but this was largely a formality. Those who accepted his authority paid him tribute every year and, as a reward, enjoyed what protection he could offer them from the other Clans they feuded with. Each summer, or battle-season, the king would try to intimidate more Clans into accepting his rulership. To do this, he needed an army. Thus enter the Fianna.
The Fianna were made up of Fir Bolg (pronounced "Fear Bollug" and literally meaning "bag men"), a nomadic race of near-giants. They never settled in any one place, and carried their belongings in a bolg, a backpack-like bag worn about their necks, from which their name is derived. The Fir Bolg were conquered many years before by the Milesians, who sailed to Ireland from the Iberian peninsula around 500 BC. It was the cunning and intelligence of the Milesians, along with their ability to settle and develop, that allowed them to achieve dominance over the physically superior Fir Bolg. The Fir Bolg made up the warrior class in the society that subsequently developed, and found themselves at the bottom of the social food chain. A woman from a higher class would not think of stooping to marry a warrior - in fact, the term "warrior's marriage" was used to describe what we now call a one night stand.
The leader of the Fianna, or Rigfennid (meaning "warrior king"), was at this time Cumhail mac Treanmhor, a member of Clan Baiscne and by all accounts a rascal and scoundrel with many enemies. Rascal or no, he lay with a beautiful woman named Muirinn (although some stories say he abducted her), and thus conceived Finn. Muirinn was no ordinary woman - she was descended from Nuadha, king of the Tuatha de Danaan (literally, "people of the Goddess Danu"), a mystical race of people who had inhabited Ireland long ago, but retreated to the Otherworld upon the arrival of the Milesians. This ancestry bestowed upon Finn some inherent affinity to magic as well as a talent for poetry, qualities which most Fir Bolg did not possess to such an extent.
Shortly before Finn's birth, Cumhail was killed in the battle of Cnucha by the men of Clan Morna, who wanted leadership of the Fianna for themselves. Many dealt blows to him, but the one who finally slew him was Aodh mac Morna, the head of Clan Morna. With his last breath, Cumhail struck out Aodh's eye and declared, "So let my murderer be marked that all men of Clan Baiscne may know him: Goll mac Morna." From then on, Aodh was known as Goll, meaning one-eye.
Now that you've got the necessary background, please continue on and read The Life of Finn mac Cumhail 2: Finn's Youth or, if you want, go back to Finn mac Cumhail.