Dwelly defines seonadh (without the "i", a related form in Scottish Gaelic) as "1. augury, sorcery.
One of their number was chosen to wade into the sea up to his waist, 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 [seaweed used as a fertilizer] for enriching our ground during the coming year.He then threw the ale into the sea, in a ceremony performed at night.
There they stood still for a time, when, on a given signal, the candle was put out, and straight-away, they adjourned to the fields where the night was spent mirthfully over the ale.
It seems likely that Seonaidh was originally some kind of god, whose worship had been lightly Christianised by the addition of various church features.