No, it was based on forensic evidence that showed several of them had handled explosives plus ‘IRA connections’ (which pretty much every Irish Catholic at the time could be said to have if you looked hard enough and defined ‘connections’ loosely enough).
There were (false) confessions but it was probably the rock solid physical evidence that convinced the jury. Also, at at least one court appearance the men showed visible injuries so it’s not like everyone didn’t know at the time that they’d been forced to confess.
Unfortunately, it turned out that the forensic evidence had been misinterpreted. I believe the expert gave evidence that the results gave 99% certainty several of them had handled explosives. In fact, the truth was that 99% of people who had handled explosives would give a positive result.
No evidence was given as to what percentage of people who hadn’t handled explosives would give a positive test (lots as it turned out). I think playing cards were shown to give a positive test result, for example.
The details above are from memory but I’m pretty sure they are correct.
On another ‘lies, damn lies and statistics’ point, the same sort of mistake did for Sally Clark. The Prosection expert took the risk of 1 cot death and squared it to get the risk of two cot deaths. This was of course complete rubbish. The babies had the same parents and lived in the same environment so if something affected one, it might well affect another.
Rock solid physical evidence and statistical analysis of the same is something the jury tend to trust 100%. Which is great until it turns out there is something wrong with the physical evidence in question.