Double Jeopardy

You can't be re-arrested for the same offence unless new evidence comes to light.

Something said in an interview would not constitute new evidence.

The OP said "outside he gives an interview" - that's not evidence.

How is this confession either "legitimate" or "verified"?

Perhaps he confessed out of bravado, or out of a desire to annoy the authorities, as some kind of revenge for being prosecuted? Perhaps he did it to make it more difficult for the state to prosecute his mate Jimmy for the crime, Jimmy being the next most likely suspect?

There are a couple of issues here, but the first one is the rule against hearsay evidence.

If I go into court, swear an oath, and testify that "I heard Walshb say that they saw HeidiHeidi stabbing A350-900", that is not admissible as evidence that HeidiHeidi stabbed A350-900. It's what lawyers call "hearsay". All we know is that there's a claim that HeidiHeidi stabbed A350-900, but we have no way of evaluating that claim. If the state wasn't to prosecute Heidiheidi for stabbing A350-900, they are going to have to bring Walshb into court to testify about what they saw.

If I go into court and say "Heidiheidi told me that she stabbed A350-900", that's also hearsay. I'm not testifying that I saw Heidiheidi stab A350-900, just that I've heard that she did.

There are many exceptions to the hearsay rule, and one exception deals with confessions. Despite being hearsay, confessions can be admitted in evidence, but there are conditions — a confession must be voluntary; it must not be obtained by threats, oppression or inducements; etc. For preference, confessions should be in writing; they should be signed; they should have been made after the accused has been cautioned and has been afforded the chance to take legal advice; etc.

So, is evidence of this confession admissible? Probably, yes. And, if admitted, it can be the "new and compelling evidence" required to get around the double jeopardy rule in Ireland? There might be a problem there. To be "compelling" for these purposes, the new evidence has to be, among other things, "reliable".

In the real world what happens after someone make a public statement admitting an offence is that he is arrested, interviewed and invited to make a formal confession, attended by the usual safeguards, in writing, signed, etc, and it's that confession that will be used in any subsequent court proceedings.

What if he's arrested, interviewed, etc and refused to make a formal confession? In that situation he's almost certainly going to repudiate the confession that he made at the courthouse door and offer a reason as to why he made a false claim to have committed the crime.

So, the issue will come down to this: is the courthouse door statement reliable? Given his account of the circumstances in which he made it, and the reasons he now offers for repudiating it, and given all the evidence adduced by the defence at the first trial to show that he didn't/couldn't have committed this offence, is this confession reliable? If it isn't, then it's not "new and compelling evidence", and there can be no second trial. That'll be the first issue thrashed out in any attempted re-prosecution.

If the confession is ruled to be reliable, then there'll be a second trial and the confession can be put before a jury. But the fact that it's reliable doesn't mean that the jury will rely on it. Remember, for him to be convicted in the second trial the jury has to be satisfied of guilt beyond reasonable doubt, so if the jury thinks there's a reasonable possibility that the courthouse door statement might have been false, the prosecution will fail. And, again, they'll be looking at all the circumstances in which he made the statement, what he said about it later in the police interview, what he says about it now, plus the evidence of the defence at the first trial. So there could be a second trial and a second acquittal.

If you saw somebody do something, your testimony about what you saw is direct evidence that he did it.

If you heard somebody say something, your testimony about what you heard is direct evidence that he said it.

If you heard somebody say "I killed X", your testimony about what you heard is direct evidence that he confessed to killing X, but it is not direct evidence that he killed X; on that point, it is hearsay.