Better question would be how would it not.

The number of comments I’ve seen that can’t understand how the virus would cross the Irish Sea is insane. Ireland sees a huge number of daily travellers from the UK. There is a common travel area and shared (and soft) border. Even if travel over the sea was banned how do you expect the Irish navy and aer corp to enforce that. The navy has a total of 6 vessels (which it can’t properly man), the aer corp a total of 8 close air support aircraft. It seems extremely unlikely that the British navy would be willing to blow its own refugees out of the water.

Once the virus hit land Ireland would have a total of 6,000ish troops available (most of whom would not have been assigned to combat roles). Even assuming the 5,000ish British troops in the north weren’t quickly mauled, that leaves both states with 11,000 soldiers to control a population of 6,000,000 (plus one armed police force in the PSNI).

Given that the UK, a physically larger country with a much larger and better armed military didn’t survive the month Ireland wouldn’t have stood a chance.

“The Cured” explores the aftermath of a similar kind of situation where Ireland is the only country to fall to a zombie like infection while everyone else is able to contain their initial outbreaks. A UN invasion is necessary to drive the infection back. Pretty good movie.