On the issue of David Cameron claiming that a decision to leave the EU would be irrevocable, it is obviously untrue as a statement in itself as JMW has pointed out.
However I think I did see him being challenged when he made that claim in a debate on one of the TV channels. As I remember it, Cameron clarified that the UK would be very unlikely to be able to go back into the EU on the current supposedly favourable membership terms (being given a rebate and being allowed to opt out of participation in the Eurozone or the Schengen Agreement), implying that if we did try to go back in, it would have to be on the unfavourable standard membership terms like adopting the Euro currency. So it sounds to me like the Conservative party has ruled out the idea of trying to take us back into the EU in the future if we left. I would imagine that the Lib Dems would try to take us back in, if they ever got the chance, even on the unfavourable standard membership terms.
If I had to predict how Brexit is going to go, I think we're likely to end up outside the EU's 'single market', due to the EU not budging on the free movement of people issue, which was a major concern for many Leave voters in the EU referendum. If the UK ends up outside the single market, then I expect that the left-leaning UK political parties, if they can get elected or can form a coalition in the future, will try to regain access to the EU's single market (rather than try to re-join the EU), in an arrangement similar to what non-EU member Norway currently has. In a Norway-type arrangement, the lefties get the freedom of movement of people back, and a lot of the EU regulations would be re-instated.