Referenda to change the constitution (thats what this is, to change the constitution to add a line "we accept the lisbon treaty") are brought by representatives of the people as bills by the members of the Dáil and/or Seanad. If this referendum was passed, another referedum could reverse the decision by removing the line. If the majority support for No was there as you say it is, any party willing to bring such a Bill (to remove the ammendment) before the Dáil and thus the irish people would receive majority support. That's a fairly tasty carrot to wave in front of Fine Gael, Labour or Sinn Fein. If a Yes vote occurs, I wouldn't rule out one of those parties adopting that very stance.
Referenda are there to poll the opinions of the voters at any one time. If the majority want a yes, it will place the ammendment into the constitution until the majority want it removed.
It's a fairly simple process when you think about it. Obviously not suited to flipping a coin though or where-ever you got the the best of three idea. Certainly not from precedence as the Nice and Divorce referenda show.