The word glen comes from the Irish gleann, meaning a narrow valley.

The word glen comes from the Irish gleann, meaning a narrow valley. Most of Ireland’s glens were carved out by glaciers during the last Ice Age as massive sheets of ice ground through the landscape leaving steep-sided valleys behind. The most famous are the nine Glens of Antrim on the northeast coast, whose inhabitants descend from both native Irish and Hebridean Scots — a legacy of the ancient kingdom of Dál Riata which once straddled both countries. The word appears all over the island in place names like Glendalough, Glenswilly and the Glen of Aherlow.​​​​​​​​​​​​​​​​