The main difference between these traditional Celtic weapons is the size and where on your person you carry your weapon.
A dagger is usually in open view, but a sgian achlais and sgian dubh are smaller and easier to conceal. Sgian achlais translates to “armpit dagger” and are worn hidden under the arm.
Dirks are an important part of any Highland dress. Historically, dirks were a part of everyday life, as both a weapon and a useful tool. Eventually, they evolved into a stylized dress accessory. Dirks are longer than a sgian dubh and, on average, are between 8 and 14 inches.
Sgian dubhs are very traditional and are almost always seen in a complete Celtic outfit. The blade of these weapons averages 3 to 4 inches. These would come in handy in everyday life for the Scotsmen of old, working in the field or a shop.