“The circumstances have changed: internationally, economically, financially and domestically. We don’t have the luxury of doing anything else. I am glad that we had a referendum. We were the only member state to do so, to have a proper debate, or something like a proper debate.”
Perhaps the "something like a proper debate" is one of the most revealing parts of this quote - he must have been quite unhappy with Libertas' direction, and he stepped down from his position there in September.
The circumstances have changed, but while they may highlight the need for reforming the EU to make it more effective, there was always the need for reform there.
He was also quoted as saying:
Both sides were guilty, he said, of “scare-mongering and misinformation” during the referendum campaign.
He's right here. There are wide ranging disputes over the validity of some of the No campaign's arguments, but the Yes side is guilty of being very vague about the risks of voting No, rather than debating the merits of the Treaty. Some on the Yes side are guilty of "playing the man rather than the ball", though Libertas doesn't seem adverse to using this strategy either.
Nunn's public support for the Lisbon Treaty is a blow to Libertas, and his position within Libertas during the referendum undermines Libertas' claim to being a grass-roots organisation whose members are independent from Ganley's business operations (their manifesto will be interesting to see if Libertas is still a one-man show [or at least Irish-directed] or more pan-European in its input and structure, rather than just a temporary anti-Lisbon alliance). Nunn also seems to accept that the guarantees can serve the purpose of allaying the public's fears and concerns, rather than aiming to start from scratch - which goes against exactly what Libertas are arguing when it comes to the legitimacy of a second referendum.
Edit: some more reaction here.