The Ale of Seonaidh is a liquid that can be used to communicate with the spirit of someone who has passed on to the Underworld. The Witch of DunBroch gifts Merida with some, so that she can communicate with her deceased father.
Before the Third Curse
Merida is able to say goodbye to her father. ("The Bear King")
After Merida defeats King Arthur and Zelena, stopping them from retrieving the enchanted helm, she takes the helm back to her castle where she is finally crowned queen. The Witch arrives to collect the helm, but Merida refuses to give it, for she believes nobody should have the power to force other's into fighting a battle they don't believe in. This decision pleases the Witch, and she reveals to Merida that she passed her secret test to see if she was a worthy leader. She gifts her with magic ale so that she can contact the one person who couldn't be at the coronation; King Fergus. Merida takes the ale to her father's grave and sprinkles it, summoning his spirit from the Underworld. Merida apologises for losing faith in her father, but he reveals he lost faith in himself. He tells her that she made him realise he if wanted to show her how to be a true leader, he couldn't use magic to do it. The father and daughter are finally able to say goodbye to each other. ("The Bear King")
After the Third Curse
A failed attempt at contacting Hook. ("Souls of the Departed")
Upon arriving in the Underworld, which looks like a demented version of Storybrooke, Emma and the others split up in order to search for Hook. However, they have very little luck, unable to locate the pirate anywhere and eventually making it so that an exasperated Gold gives up doing things their way and heads to his shop to retrieve something that might help. He returns with the Ale of Seonaidh which, when poured over someone's grave, will summon that person's spirit and allow for contact to be made. If they pour it over Hook's, he will be able to tell them where he is and they can leave within the hour. The others are surprised by the prospect of Hook having a grave down there, but Gold soon reveals that everyone does, and they quickly embark upon a graveyard that's acres upon acres big. Finally, they locate the grave of Killian Jones and Emma empties some of the ale over it. Nothing happens at first, but then a vision flickers to life, and Hook is seen... although he can't see them. They are shocked to see that he is all bruised and bloody and burnt, clearly undergoing some pretty intense torture, and Emma is distraught when the spell is unable to hold and he vanishes. Regina later uses the ale on her father's gravestone to contact him and apologise for everything she did to him. ("Souls of the Departed")