Which 7th-century Irish missionary, a companion of Columbanus, founded the monastery of St. Gall in Switzerland? Gall.
Gall, also called Gallus, was an Irish monk and missionary who accompanied Columbanus during missions across the Frankish kingdoms. Around 612, he established a hermitage in the Steinach valley near Lake Constance, in what is now northeastern Switzerland. That solitary settlement eventually gave its name to the city and abbey of St. Gallen.
Gall’s original foundation was a hermitage rather than the fully organized medieval abbey later associated with the site. After his death, disciples remained near his cell and cultivated a religious community. The monastery was formally developed in the 8th century, especially under Saint Othmar, who became its first abbot and introduced the Rule of Saint Benedict.
The Abbey of Saint Gall later became a major center of learning, manuscript copying, music, and scholarship. The “Gall” in the answer can be confusing because English sources also use Gallus, while the modern city is usually called St. Gallen. Columbanus was Gall’s missionary companion, not the founder named in this question.