Someone said :
Did ye hear the gardai are hunting down people who had contact with the man in Dublin infected? What the hell are the gardai gonna do?
They can access phone records, and find them, notify to them the risk and the public health recommendations (probably based on voluntarily complying, at first).
They have the skills and tools to trace people.
"Hunting down" is the sensationalist headline: they are just tracing people, centralizing and recouping information... in other words the basic detective work needed.