Former CIA Director John Brennan said the agency first suspected that the Russians were "getting more and more involved" in efforts to influence the outcome of the 2016 presidential election last spring.
Speaking on RTÉ's Today with Sean O’Rourke, he said that by the summer those efforts had become quite complex and far reaching, which is when the FBI launched its investigation.
Mr Brennan pointed to the growth of cyber domains and said this provided more opportunity for "trouble making".
He said this was not just isolated to the United States but that Russia had been heavily involved in trying to exploit elections in Europe over the past several decades.
Mr Brennan added it was time to say "no more" and push back heavily against the Russians.
He described former FBI director James Comey as a great patriot who was non-partisan.
Mr Brennan said he had worked with six presidents but had never been asked to show or give loyalty to the president.
Being asked to do so, he said, would have "set off Jim Comey's radar".
When asked if he could have worked with President Trump, Mr Brennan said "I was finished at noon on inauguration day".
He said he had been privileged to work with six presidents, with whom he did not always agree, but felt they were trying to do their best for the country and not themselves, adding, "I think that my time was best finished on 28th January."
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Separately, Mr Brennan said no country should feel immune from a terrorist attack or having terrorists use their country for "whatever purposes".
He said he knew the Irish leadership was aware of the need to "stay vigilant" and that British police have close relationships with gardaí, frequently sharing information.
Mr Brennan added that we should not be fearful to the point of limiting freedom and liberties that were hard fought for in the West.
He praised counter terrorism officials across the world who, he said, have stopped a lot of attacks.