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.