In Ireland, the saint is commonly known as Colmcille. Colmcille is one of the three patron saints of Ireland, after Patrick and Brigid of Kildare.

Columba is historically revered as a warrior saint and was often invoked for victory in battle. He left Ireland for Scotland not as a missionary but as an act of self-imposed penance for a bloody mess he had caused at home.

The Battle of Cul Dreimhne
The Battle of Cul Dreimhne: sometime around 560, the Irish abbot and missionary Saint Columba became involved in a quarrel with Saint Finnian of Movilla Abbey over a psalter. Columba copied the manuscript at the scriptorium under Saint Finnian, intending to keep the copy. Saint Finnian disputed his right to keep the copy. Thus, this dispute was about the ownership of the copy (whether it belonged to Saint Columba because he copied it or whether it belonged to Saint Finnian because he owned the original). King Diarmait mac Cerbaill gave the judgement, "To every cow belongs her calf, therefore to every book belongs its copy."

Columba disagreed with King Diarmait's ruling against him and reportedly instigated a successful rebellion of the Ui Neill clan against the King. The battle was claimed to have caused around 3,000 casualties.

St. Columba and the Isle of Iona
As a self-imposed penance, Columba settled on the Isle of Iona. In 563 AD, St. Columba arrived on its white sandy beaches with 12 followers, built his first Celtic church and established a monastic community. The Isle of Iona is three miles long by one mile wide, but has had an influence out of all proportion to its size on the establishment of Christianity in Scotland, England and throughout mainland Europe. Full of remorse for his actions and the deaths he had ultimately caused, he settled on Iona as the first place he found from where he couldn’t see his native Ireland. One of the features on the island is even called "The Hill with its back to Ireland".

What miracles did St Columba perform? (Book Two of his Miraculous Powers)
In the second book, Columba performs various miracles such as healing people with diseases, expelling malignant spirits, subduing wild beasts, calming storms, and even returning the dead to life.

From the Gollaher Family Foundation:
St. Columba (Columbcille in Gaelic, meaning “dove of the church”). Grandson of Niall of the Nine Hostages. Apostle of the Northern Picts, born. 7 Dec. 521 in Co. Donegal, he spent his formative years not among his family but with his foster father, the holy man Cruithnechan, in Leinster. Founded the church of Derry (now Londonderry) 545, and many other monastic churches, also the monastery of Durrow 553, caused a war in which his kinsmen the Northern Ui Neill defeated the King of Ireland (Diarmaid, head of the Southern Ui Neill) 561, went into voluntary exile, founded the Abby of Iona 563, converted Bruide, King of the Picts; inaugurated his own cousin Aidan as King of the Scots of Dalriada 574, and died June 9, 597.

Revered second only to St. Patrick Columba left an incredible theological mark across Ireland and Scotland. The young Columba spent some time at the monastery in Moville, Ireland. Intrigued by the plentiful books there, he began to make a copy of one of the psalters but was caught in the act by the abbot, who considered this akin to stealing the actual book, and he appealed to King Diarmaid for judgment. In what was probably the world’s first copyright lawsuit, the king decided the case in favor of the abbot, saying “as the calf is to the cow, so the copy is to the book.” Infuriated at this ruling, Columba followed a decidedly unsaintly course. He rallied his kinsmen and engaged the king’s army in a battle in which more than 3,000 men were killed.

Brought before the religious and royal authorities to face punishment for his instigation of the conflict, but miraculously managed to escape serious penalty. At the church trial, he reportedly was preceded into the room by a column of white light, a portent that the church elders determined to be a sign from above. Not wanting to defy a divine omen, they decided not to excommunicate him. When brought before the royal court in the year 563, the king also showed leniency and did not have him executed, but instead exiled him. Remorseful for causing so many deaths, Columba solemnly vowed to the Irish leaders that he would convert one person for each one that had died during the battle, and went on to found the famous Abby of Iona and become the most beloved saint of all Scotland.

In 575 Columba returned to Ireland on a peace-keeping mission, and while there defended the rights of the non-Christian bardic poets against Aed, son of King Ainmere, who had ordered their banishment.

Conal Ghulban
First son of Niall of the Nine Hostages, Conall Ghulban, King of Tír Chonaill or the “Land of Conall” (Tyrconnell or Tirconnell in anglicized English), the lands to the west of Aileach, which was his share of the family’s conquests in north-western Ulster after 425. His descendants, known as the Cenél Conaill, formed one of the principle branches of the Northern Uí Néill, and until the 12th century their kings were inaugurated at the sacrifice of a white mare, going down on all fours like a stallion and lapping its broth. As the kindred of St. Columbia, members of this branch were also Abbots of Iona 563-891 or later, Abbots of Dunkeld from the 9th to 12th centuries, and Kings of Scots from Duncan I (slain by MacBeth 1040) to Alexander III (died of a fall from his horse 1285/86).