The Ale of Seonaidh is based on the item of the same name from Celtic mythology.
Before Third Curse
After she passes the test and proves herself as a ruler, the Witch awards Merida with some of the ale and explains its use. Merida uses the ale on the tombstone of her father. When the first splash doesn't appear to work, she tries again but is stopped by Fergus's spirit, who tells her not to waste good ale. He congratulates her and they converse shortly before saying one final goodbye and his spirit vanishes. ("The Bear King")
After Third Curse
In the Underworld, Mr. Gold takes some of the ale from the Underworld version of his pawnshop and gives it to Emma Swan so that she may find the location of her true love, Hook, and they can leave. However, when Emma splashes the ale on Killian's tombstone, the spell is blocked partially and Killian is distorted slightly and unable to hear them or tell them where he is. The effect is also cut short. ("Souls of the Departed")
Seonaidh is a Celtic water spirit from the Isle of Lewis, who had to be offered a cup of ale in hope of prosperity in the forthcoming year.