Mug Ruith (or Mogh Roith, "slave of the wheel") is a figure in Irish mythology, a powerful blind druid of Munster who lived on Valentia Island, County Kerry. He could grow to enormous size, and his breath caused storms and turned men to stone. He wore a hornless bull-hide and a bird mask, and flew in a machine called the roth rámach, the "oared wheel". He had an ox-driven chariot in which night was as bright as day, a star-speckled black shield with a silver rim, and a stone which could turn into a poisonous eel when thrown in water.

Stories about Mug Ruith are set in various periods of Irish history. Some say he lived during the reign of 3rd century High King Cormac mac Airt, while others put him in Jerusalem during the time of Christ. In Lebor Gabála Érenn he is said to have died in the reign of Conmael, nearly two thousand years before Cormac's time. Perhaps due to this array of times and settings, poets attributed the druid with extraordinary longevity (he lived through the reign of nineteen kings according to one story). His powers and long lifespan have led some to conclude he was a euhemerised sun or storm god.

A mug is a sturdily built type of cup often used for drinking hot beverages, such as coffee, tea, or hot chocolate. Mugs, by definition, have handles and often hold a larger amount of fluid than other types of cup. Usually a mug holds approximately 12 fluid ounces (350 ml) of liquid; double a tea cup. A mug is a less formal style of drink container and is not usually used in formal place settings, where a teacup or coffee cup is preferred. Shaving mugs can be used to assist in wet shaving.

Whereas ancient mugs were usually carved in wood or bone or shaped of clay, most modern ones are made of ceramic materials such as earthenware, bone china, porcelain or stoneware. Some are made from strengthened glass, such as Pyrex. Other materials, including plastic, steel and enameled metal are preferred where break resistance and reduced weight are at a premium, such as for campers. Techniques such as silk screen printing or decals are used to apply decorations; these are fired onto the mug to ensure permanence.