I would say that a coat of arms is never an honor, and while the College of Arms, Lyon Court, and the Chief Herald of Canada disagree with me, the Chief Herald of Ireland does not. So not only is an Irish grant of arms not Ireland's highest honor, it's not an honor at all.

But it is not "honors" that are prohibited by the Constitution, but presents, emoluments, offices, and titles. A coat of arms is obviously not an office or title. If granted without cost, it is a present or emolument. But the law permits a government official, including the president, to accept presents or emoluments below a certain value (which changes over time) for himself. Above that value, they become the property of the government.

I would say that a grant of arms consists of two parts: the exclusive right to use the arms, and the document (letters patent) memorializing that right. The right to use the arms has no market value because once granted it cannot be sold, so it obviously falls below the legal threshhold. The letters patent for both Kennedy and Clinton belong to the U.S. National Archives and are housed at their respective presidential libraries. No problem.