Being a jurist, I'll start with the annoying lawyerly answer: Who would have standing to sue over a breach of promise? All citizens? All voters? Or just the people that voted for the party in question? As a matter of normal private law, the answer would be that only the latter category would qualify, since they are the only ones who provided consideration, and are therefore a party to the "contract". But, given that elections are secret, how are people supposed to prove who they voted for?
Giving the right to sue to people who did not vote for the party in question opens the system up to massive abuse, because the party would have to endlessly defend itself in court against law suits by its political enemies. That would be enormously costly in terms of time and money, and even if the political parties were somehow spared the cost of defending themselves, it would still involve dragging the people who are supposed to run the country (and only they, since they are the only ones in a position to break their promise) into court over and over again. That can't be good.
More generally, I think we don't want politicians to keep their promises more than they currently do. Jason seems to envisage a system where politicians are much more careful about the kinds of promises they make, which is already better than an outcome where politicians keep their promises even though it is against the best interest of the country, but in practice those two alternatives will be impossible to separate. No one can know what will happen during the next 4 or more years and anyone who sets out to limit themselves to promises they know they will be able to keep will end up with a very short list of promises.
And this mistakes the purpose of these promises. Politicians don't make promises as part of some bargain with their voters, as a quid pro quo. They make promises to signal their political philosophy, their ideology. The promises signal how the politician will behave with regard to a million other issues that he can't and won't list.
Finally, I don't think Jason's solution for coalition building is feasible. Whatever merit his idea has in two-party systems like the US or the UK, it is much less desirable in a country like mine where we may well end up with a five-party coalition after September. Going into such negotiations with a list of promises that cannot legally be reneged upon makes coalition-building all but impossible. This is all the more true because the effect of the system will be for politicians to make more such promises than they optimally should; it's a prisoners' dilemma where everyone ends up making too many promises.
(Every promise you make gives you a higher payoff in terms of votes, while the cost of the promise is in terms of bargaining problems, i.e. shared between you and the other political actors, as well as the public at large. Result: everyone makes suboptimally many promises.)
So instead, let's turn to another Irishman: Edmund Burke:
... it ought to be the happiness and glory of a representative to live in the strictest union, the closest correspondence, and the most unreserved communication with his constituents. Their wishes ought to have great weight with him; their opinion, high respect; their business, unremitted attention. It is his duty to sacrifice his repose, his pleasures, his satisfactions, to theirs; and above all, ever, and in all cases, to prefer their interest to his own. But his unbiased opinion, his mature judgment, his enlightened conscience, he ought not to sacrifice to you, to any man, or to any set of men living. These he does not derive from your pleasure; no, nor from the law and the constitution. They are a trust from Providence, for the abuse of which he is deeply answerable. Your representative owes you, not his industry only, but his judgment; and he betrays, instead of serving you, if he sacrifices it to your opinion.