"The violence was wrong on all sides and I've said so all along," he told the BBC's Andrew Marr Show.
"My whole point was that if you were to bring about a peace process, you weren't going to achieve it by military means."
He said his message was "there was never going to be a military victory so there has to be a political dialogue".
Mr Corbyn said that at the same time he was making himself "very unpopular with some people through my preparedness to reach out to the republican tradition in Ireland", the government was also holding secret talks.
He said the Northern Ireland peace process was a "great achievement".
Asked about his support for a united Ireland, he said: "It's for the Irish people to decide - my view is historically, yes, and I'm very much on the record about that.
"Quite honestly, the peace process has brought about a huge step forward - there is a lot of cross-border agreement, there are a lot of cross-border institutions.
"You go to Belfast, you go to Dublin - people travel back and forth all the time. The governments are in touch with each other every hour of every day on different issues.
"There is a sense that there is one island of Ireland."
Mr Corbyn was speaking in Brighton ahead of his first annual conference as party leader.
Democratic Unionist Party MLA Arlene Foster said Mr Corbyn had been a "cheerleader" for "those who wished to break up the United Kingdom by force".
"So few people, either in Northern Ireland or the rest of the United Kingdom, appear to believe the revisionism and excuses offered about his actions in the past," Northern Ireland's acting first minister added.
Labour's shadow Northern Ireland secretary Vernon Coaker told BBC Radio Ulster's The Sunday News programme the party's policy "remains the same" as it was prior to Mr Corbyn's election as leader.
"[Our policy] will build on the Good Friday Agreement and subsequent agreements," he said.
"We have no wish and we will make no effort to change that, notwithstanding Jeremy Corbyn's remarks in the past or his repetition of one or two of those."
Labour Party leader Jeremy Corbyn has defended his controversial talks with Irish republicans during the Troubles.