The new clause in the constitution, if accepted, will say:
That paragraph may look innocuous but read it carefully; No provision of this Constitution invalidates laws enacted, acts done or measures adopted... So the treaty completely overrides the constitution; the rest of the constitution becomes subordinate to the treaty. Then, or prevents laws enacted, acts done or measures adopted by bodies competent under that Treaty from having the force of law in the State... So a "measure adopted by a body" has the force of law? What happened to democracy, to elected representatives making the laws? That has all gone overboard. In the future "bodies" will make the law.
The main reason the powers-that-be want a "yes" vote is because the two European bailout mechanisms (the European Financial Stability Mechanism, EFSM, and the European Financial Stability Facility, EFSF) are due to be merged into a single European Stability Mechanism, ESM, by March next year and getting money out of that will be conditional on having ratified the treaty. So Ireland's national budget would be blown out of the water if they vote no. They are hoping to borrow about €17bn next year.
So in a nutshell the options are: give up independence and get money, or keep independence and get no money.
All the opinion polls show a very large proportion of the Irish electorate are "undecided" so it's not clear what the outcome will be. This blog opines that it will probably be "yes" by a whisker, but it is possible the Irish will discover they have a backbone.