The temptation to tell someone that they are being “irrational” or “unreasonable” can be hard to resist, but usually resisting is exactly what you should do.
The problem with accusing someone of irrationality is that it can seem you are expelling their position from the domain of the rational, as though it does not even merit rational consideration. This is often offensive, which does not facilitate sensible and constructive dialogue.
When we say someone is irrational we could mean one of two things. One is that their argument strives to be rational but it is not a good one. In such cases it is strange to claim that the speaker is irrational. All the great philosophers have put forward arguments that fail, but we do not generally cal them irrational. So why dismiss others whose arguments do stand tough scrutiny as irrational? Isn’t it enough to say why we think they are wrong?
The stronger sense of “irrational” is someone who is not even attempting to offer a rational argument. People are irrational in this sense much less often. We should at least assume that someone is trying to rational and engage with them as someone committed to reason unless it is absolutely clear that they are not.
If someone really is rejecting reason, pointing this out is hardly likely to bother them. In such cases, it is useful to clarify what role, if any, the person gives to reason. The discussion then has to move to another level: to discuss the legitimacy of holding beliefs on non-rational grounds, and you can have a rational argument about that. There could be good rational reasons for accepting some non-rational grounds for belief.
The key point is that if we object to an argument by saying it is not rational, you either wrongly accuse the other person of having abandoned reason when really you simply think they are no reasoning well; or you are making what seems like a criticism to you but has no sting for the other person. Better then to avoid levelling the charge at all and stick to discussing the specific merits and flaws of the argument. —Julian Baggini