Simply saying something is irony does not mean the writer was successful in conveying the intent. That is when it comes across as sarcasm.

Two ways irony that isn't meant hurtfully can miss the mark - one is when it is taken literally. The other is when it is taken as being mean ironically, but understood to be intended to hurt, ie sarcasm.

But very often people will use a claim that they have been speaking ironically as a defence when someone objects to some offensive remark which wasn't meant ironically at all. That has the advantage of implying that the person objecting is lacking in wit or in a sense of humour.