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The birlinn (spelt bìrlinn in Scottish Gaelic) was a type of boat used especially in the Hebrides and West Highlands of Scotland in the Middle Ages. Variants of the name in English and Lowland Scots include "berlin" and "birling". The Gallo-Norse term may derive from the Norse byrðingr, i.e. a ship of burden. However, a local design lineage can be traced to the Broighter-type boat of Dál Riata, equipped with oars and a square sail, dating from the first century BC, without the need to assume a specific Viking design influence. The birlinn, like its larger counterpart, the long-fhada ("long-ship"), was clinker-built and could be sailed or rowed. It had a single mast with a square sail. Smaller vessels of this type might have as few as twelve oars, with the larger West Highland galley having as many as forty. For over four hundred years, down to the seventeenth century, the birlinn or its larger variant, the long-fhada, was the dominant vessel in the Hebrides. The birlinn appears in Scottish heraldry as the "lymphad" (a corruption of long-fhada).
In terms of design and function, there was considerable similarity between the local birlinn and the ships used by Norse incomers to the Isles. In an island environment ships were essential for the warfare which was endemic in the area, and local lords used the birlinn extensively from at least the thirteenth century. The Lords of the Isles of the Late Middle Ages maintained the largest fleet in the Hebrides. It is possible that vessels of the birlinn type were used in the 1156 sea battle in which Somerled, Lord of Argyll, the ancestor of the said lords, firmly established himself in the Hebrides by confronting his brother-in-law, Godred Olafsson, King of the Isles.