In the non-ironic sense (“I'm about to say something important”), any method for emphasis will do. For example:
- “Tenez-vous le pour dit : je ne viendrai pas.” (lit. “Hold that it has been said”)
- “Qu'on se le dise : je ne viendrai pas.” (lit. “Let it be said”)
- “Je vous l'affirme : je ne viendrai pas.” (lit. “I assert it in front of you”)
Since George Bush used the phrase in 1988 and his political opponents later pointed it out as a lie, the phrase has acquired an ironic bent. Any of the phrases above can be used ironically, but context is required to make the irony apparent, you can't rely on the cultural implication like in English.
If the implication is “everyone knows that what I'm about to say is a lie”, there is a somewhat similar phrase, of similar vintage, from a TV debate between the two main candidates for the the 1988 French presidential elections. The original exchange was:
Jacques Chirac — Pouvez-vous vraiment contester ma version des choses en me regardant dans les yeux ?
François Mitterrand — Dans les yeux, je la conteste.
— Can you really dispute my presentation of the facts, eye to eye?
— Eye to eye, I dispute it.
It was widely known that Mitterrand was lying, but during the debate, he didn't let on. Since then, at least in France, the phrase “Dans les yeux, …” or more often the misquoted phrase “Droit dans les yeux, …”, has become a way to say “I affirm the following thing, even though you and I know that I'm lying”. Common variations include “droit dans les yeux, je vous le dis” or “droit dans les yeux, je vous l'affirme”. This is probably not known outside France and may not be known to younger generations who weren't following politics in 1988.