Earlier this month Irish police intercepted a suspected dissident republican haul of rocket launchers and explosives in Co Tipperary.
It is believed the arms were en route to Northern Ireland.
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Authorities have warned that the death toll is likely to rise as much of the building is yet to be searched.
The PM will sound out the possibility of new free trade agreements with Gulf states for once the UK has left the EU.
In the unexpected announcement on Monday, Mr Key said it was the "right time" to leave politics.