Saint Kessog, also known as Saint Kessoc or MacKessog, lived from about 460 to 10 March 520, though there is some doubt about these dates. We have seen sources that give his date of death as 10 March 560, which would mean his year of birth must also have been later. We've gone with the majority view in giving 460-520. Kessog was born into the royal family of Munster in Ireland and made his name as a missionary in Scotland. The wider picture in Scotland at the time is set out in our Historical Timeline.
Kessog was the son of the King of Cashel in Munster and started his religious life while still in Ireland. It is said that a swimming accident when he was a child led to the deaths of the sons of a number of visiting princes. Kessog brought them back to life and averted a war by spending a night in prayer. He was then educated at a monastery by St Patrick and St Machaloi before setting out for Scotland.
St Kessog was mainly active in west and central Scotland, having established a monastery on the island of Inchtavannach (Monk's Isle) on the western side of Loch Lomond. He was also active across southern Perthshire. He was attacked and killed at Bandry, on the western shore of Loch Lomond overlooking Inchtavannach, and that the place was marked by St Kessog's Cairn. The reasons for his murder are unclear.
Kessog was buried on the western shore of Loch Lomond, and the herbs that grew up around his grave led to the place becoming know as Luss (Gaelic for "place of herbs"). St Kessog was widely venerated in the medieval period. Troops under Robert the Bruce used "Blessed Kessog" as a battle cry during the Wars of Independence, and he was considered to be the patron saint of Scotland until Saint Andrew took over the role.
St Kessog is remembered in the name of a number of churches, including St Kessog's Church in Luss, in which there is an effigy of the saint. The old parish churches in Comrie, Callander and Auchterarder are also named after St Kessog, as, rather further afield, are the villages of North Kessock and South Kessock near Inverness. It follows that the Kessock Bridge, and before it the Kessock Ferry, are/were named after St Kessog, as is the Kessog oil field in the North Sea.