Could It Happen Again?

When a bomb exploded in Omagh, a market town in the heart of Northern Ireland, five years ago this month, Michael McKevitt was puttering around in the garden of his home on the outskirts of Dundalk in the Irish Republic. Within minutes of the blast, he received a phone call informing him that a number of civilians had been killed. That was an understatement. Twenty-nine people died in the Omagh atrocity, the most deadly incident in more than 30 years of the Troubles.

This month McKevitt was convicted of directing the Real I.R.A., a republican splinter group, and...