The Queen is to make history by becoming the first British monarch to visit to Ireland for 100 years.
The visit is being conducted under tight security, and one bomb has already been discovered and made safe on a bus outside the Glen Royal Hotel in Maynooth, County Kildare.
Former Met assistant commissioner Andy Hayman said the attempted attack vindicated the high cost of security surrounding the Queen.
"What we're seeing of the intelligence now is coming true. Even though it's very worrying and disappointing," he told John Humphrys.
The "dual pronged attack" of extremist international terrorism and dissident Irish terrorism was causing a "real headache" for security services.
The terrorist threats are "deeply disappointing", Irish broadcaster Ryan Tubridy said, around an event that "should have been about looking forward".
With a major security operation in Dublin centre, he said, the Queen would be well looked after on her visit, despite some ambivalence from the Irish public.
"Will the Queen be welcome? Of course. Will we be waving Union Jacks in the street? It's a bit soon for that," he added.
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