'Cú' literally means 'dog' or 'hound' in Irish, while more specifically he is an Old English Sheepdog.
In one of the scenes where Saoirse and Ben are walking in the city, a close-up of the iconic statue of Molly Malone can be seen briefly, thus it could be inferred that they're walking in the streets of Dublin, Ireland.
One of the trick-or-treaters on the bus is holding a turnip Jack-o'-lantern. Original Celtic Halloween celebrations carved the lanterns out of turnips and various other root vegetables.
The ferry is named "Ceol na Mara", which is Irish for "Music of the Sea".
In the bus scene you can see Aisling the fairy from Tomm Moore's previous film The Secret of Kells (2009) in the background. The two films may occupy the same universe, so it could actually be Aisling wandering the modern world on Halloween.