I suppose there is a failure of logic with the borders. It is true to say that the virus was imported from abroad and that then spread like wildfire. It seems that the acts of travel, gathering and socialising that effects the incidence and prevalence. Essentially humans living a normal life, working, communicating and seeking social interaction with other humans is the vector for this virus to spread.
the time to close the borders was in March, once it was it it was too late. Now the Irish government seems to be trying to eradicate the virus, which isn't going so well either.