Seonaidh was according to Martin Martin

Martin Martin

Martin Martin was a Scottish writer best known for his work A Description of the Western Isles of Scotland . This book is particularly noted for its information on the St Kilda archipelago...

, the name of a water spirit in Lewis.

Dwelly defines seonadh (without the "i", a related form in Scottish Gaelic) as "1. augur

Augur

The augur was a priest and official in the classical world, especially ancient Rome and Etruria. His main role was to interpret the will of the gods by studying the flight of birds: whether they are flying in groups/alone, what noises they make as they fly, direction of flight and what kind of...

y, sorcery

Magic (paranormal)

Magic is the claimed art of manipulating aspects of reality either by supernatural means or through knowledge of occult laws unknown to science. It is in contrast to science, in that science does not accept anything not subject to either direct or indirect observation, and subject to logical...

. 2. Druid

Druid

A druid was a member of the priestly class in Britain, Ireland, and Gaul, and possibly other parts of Celtic western Europe, during the Iron Age....

ism" and quotes Martin further.

Martin says that the inhabitants of Lewis used to propitiate Seonaidh by a cup of ale

Ale

Ale is a type of beer brewed from malted barley using a warm fermentation with a strain of brewers' yeast. The yeast will ferment the beer quickly, giving it a sweet, full bodied and fruity taste...

 in the following manner. They came to the church of St. Mulway (Mael rubha), each man carrying his own provisions. Every family gave a pock (bag) of malt

Malt

Malt is germinated cereal grains that have been dried in a process known as "malting". The grains are made to germinate by soaking in water, and are then halted from germinating further by drying with hot air...

, and the whole was brew

Brew

Brew may refer to:* Brew , a Melbourne Cup winner in 2000* Brew House Association, an artistic collective on the south side of Pittsburgh* Brew Records, a record label producing UK-based band Hawk Eyes* Drip brew, a method for brewing coffee...

ed into ale. One of their number was chosen to wade into the sea

Sea

A sea generally refers to a large body of salt water, but the term is used in other contexts as well. Most commonly, it means a large expanse of saline water connected with an ocean, and is commonly used as a synonym for ocean...

 up to his waist

Waist

The waist is the part of the abdomen between the rib cage and hips. On proportionate people, the waist is the narrowest part of the torso....

, carrying in his hand the cup full of ale. When he reached a proper depth, he stood and cried aloud:

"Seonaidh, I give thee this cup of ale, hoping that thou wilt be so good as to send us plenty of seaware for enriching our ground during the coming year."

He then threw the ale into the sea. This ceremony was performed in the night

Night

Night or nighttime is the period of time when the sun is below the horizon. This occurs after dusk. The opposite of night is day...

-time. On his coming to land, they all repaired to church, where there was a candle burning on the altar. There they stood still for a time, when, on a signal given, the candle was put out, and straight-away, they adjourned to the fields

Field (agriculture)

In agriculture, the word field refers generally to an area of land enclosed or otherwise and used for agricultural purposes such as:* Cultivating crops* Usage as a paddock or, generally, an enclosure of livestock...

 where the night was spent mirthfully over the ale. Next morning, they returned to their respective homes, in the belief that they had insured a plentiful crop

Crop (agriculture)

A crop is a non-animal species or variety that is grown to be harvested as food, livestock fodder, fuel or for any other economic purpose. Major world crops include maize , wheat, rice, soybeans, hay, potatoes and cotton. While the term "crop" most commonly refers to plants, it can also include...

 for the next season.

It seems likely that Seonaidh was originally some kind of god

God

God is the English name given to a singular being in theistic and deistic religions who is either the sole deity in monotheism, or a single deity in polytheism....

, whose worship

Worship

Worship is an act of religious devotion usually directed towards a deity. The word is derived from the Old English worthscipe, meaning worthiness or worth-ship — to give, at its simplest, worth to something, for example, Christian worship.Evelyn Underhill defines worship thus: "The absolute...

 had been lightly christian

Christian

A Christian is a person who adheres to Christianity, an Abrahamic, monotheistic religion based on the life and teachings of Jesus of Nazareth as recorded in the Canonical gospels and the letters of the New Testament...

ised by the addition of various church features. However, it is also possible that "Seonaidh", which is the Scottish Gaelic form of the English Johnny

Johnny (given name)

Johnny is a masculine and feminine given name, and a masculine pet name, in the English language. It is sometimes a pet form of the masculine John, and since the 16th century it has also been a standalone masculine given name. In some cases Johnny is also a feminine given name. Variant forms of...

, may also be a reference to one of the Saints John, and some kind of invocation to him.