Real world, the tale of St Patrick banishing snakes from Ireland is a reference to the Draconis, the war banner/streamers, and refers to him bringing peace to the island. But in Dresdenverse, you could well have a race of snake-demons who've been banished from their home and want to come back.
The Giant's Causeway might allow travel to Scotland; or the group might be asked to repair it - allowing invasions of giants.
Ireland is surprisingly young; when the ice retreated, Ireland was just a flat piece of rock. Winter might resent this johnnie-come-lately nation.
Ireland is surrounded by mythical nations; Hy Brasil to the west, Ys and Lyonesse to the south, Thule to the north, Cantre'r Gwaelod (and Sodor!) to the east. For that matter, the Isle of Man is important in the Arthurian myths as the home of the Fisher King.
In Britain, Arthur pulls the sword out of the stone; in Ireland, Cúchulainn drove his sword *into* the kingstone, the Lia Fáil. The Scots insist that their king stone came from ireland, the Irish that their came from Scotland - that has potential for lunacy. Suppose the stone sang again when a (n)pc approached it? How would the PCs reacted to knowing who the rightful king of Ireland was?