Here are some possibilities, with explanations of their nuances:
* Scottish Gaelic:
* Rèidh: This is a good general term for a smooth *surface*. Think of a smooth table or smooth skin.
* Siobhalta: This describes something that is *easy*, *fluent*, or *smooth* in its progress or movement. For example, a smooth journey.
* Bog: This means soft or yielding, and could be used for a smooth, soft texture.
* Irish Gaelic:
* Réidh: Similar to the Scottish Gaelic word, this is a common word for a smooth *surface*.
* Sleamhain: This means slippery, and while not directly "smooth," it could be used to describe a smooth, slippery surface.
* Réidh/Bog: Similar meanings as their Scottish Gaelic counterparts. `Bog` carries the softness aspect.
To get the most accurate translation, you need to specify what you want to describe as "smooth."