I don't think this is primarily about Chaim Herzog himself, it's about Israel.
Ronald Reagan was hardly faultless, and it's entirely reasonable to suggest Reagan was partly responsible for some of the USA's actions at the time he was US President.
Israel has a decades-long history of contempt for both International Law and Human Rights. This doesn't just stem back to Herzog's time, it predates it all the way back to the advent of the Israeli State. If you continually conduct yourself in similar ways to other pariah states, I don't think it's any wonder then that eventually other nations stop taking your claims to be an upstanding member of the international community on face value, and eventually they begin to view you and treat you in no different a way to they do those other pariah states.
Israel's recent excesses may have served to prompt this change, but it's not as if Israel's reprehensible behaviours only begun in the aftermath of the October 7th attacks. The period since is the straw that broke the camel's back, so to speak.
It's worth noting that Ireland has been one of Israel's most strident critics of late. In my opinion it's only natural to expect elected governments to follow public opinion to a degree, and if there is a distaste in the population for honouring prominent members of what is regarded as a pariah state, I'd expect that to be reflected in the decisions then taken.
If the US fell into disrepute in the wider international community to the extent that Israel has, I expect there would likely be people also railing against the honouring or veneration of Ronald Reagan. Some still do anyway, just for slightly different reasons to those related to Israel.