The anti treaty side have argued that a no to Lisbon will mean 2 things.
1. If one country votes no then it is legally dead and buried and it does not matter if the other 26 ratify it.
2. We now default back to the Nice treaty which has no provisions for expelling any of the 27 members.
They are opposed to any further political integration of Europe.
Basically all along they have been saying vote no and nothing will happen things will simply continue as they were before and to a certain extent they have, all the dire predictions from the Yes camp have come to nothing (so far).
Actually, contrarily to what many think, this is not true.
1. Nothing prevents the 26 countries from ratifying the treaty, leaving Ireland outside of it, and thus creating a new EU. Let's call it the Lisbon-zone (just like we already have a Schengen-zone and a euro-zone). Tha fact that Ireland is an island and lies at the fringe of the EU makes it all the easier to implement this solution. A Schengen-zone without France or Germany would be unthinkable, but without the UK and Ireland (as it is now), it works fine, justly because these are islands.
2. The Nice Treaty may not have provisions to expel a member state, but nothing prevent some countries to move ahead with deeper integration before the others. That's how the EU has worked so far (visas, euro, EEC...).
Indeed, if the Irish people rejects again Lisbon, nothing will change for them, but other countries will just move on with further integration, leaving Ireland with less political influence inside the EU.