Not grounded in reason. (Compare rational.)
Jung pointed out that elementary existential facts fall into this category-for instance, that the earth has a moon, that chlorine is an element or that water freezes at a certain temperature and reaches its greatest density at four degrees centigrade-as does chance. They are irrational not because they are illogical, but because they are beyond reason.
In Jung’s model of typology, the psychological functions of intuition and sensation are described as irrational.
Both intuition and sensation are functions that find fulfilment in the absolute perception of the flux of events. Hence, by their very nature, they will react to every possible occurrence and be attuned to the absolutely contingent, and must therefore lack all rational direction. For this reason I call them irrational functions, as opposed to thinking and feeling, which find fulfilment only when they are in complete harmony with the laws of reason.[Ibid., pars. 776f.]
Merely because [irrational types] subordinate judgment to perception, it would be quite wrong to regard them as "unreasonable." It wouldbe truer to say that they are in the highest degree empirical. They base themselves entirely on experience. ["General Description of the Types," ibid., par. 616.]