Irrealis is a type of verb form used when speaking of an event which has not happened, is not likely to happen, or is otherwise far removed from the real course of events. For example, in the sentence "If you had done your homework, you wouldn't have failed the class.", had done is an irrealis verb form. A verb is a part of speech that usually denotes action (bring, read), occurrence (to decompose (itself), to glitter), or a state of being (exist, live, soak, stand). Depending on the language, a verb may vary in form according to many factors, possibly including its tense, aspect, mood and voice. ...
Irrealis prefixes are used in conjunction with grammatical markers indicating negation, prohibition, obligation, conditional, etc and also in conjunction with simulative, infrequentative and admirative prefixes.
Irrealis is used for i) future events ii) commands, exhortations and warnings iii) counterfactual events iv) sequences of customary and habitual events.
E.g., the 'future' 'irrealis' in Kiowa has a number of uses that are associated with irrealis in other languages, but clearly it does not belong to a binary system or even a trinary system (of realis, irrealis and unmarked).
Irrealis here means the formal marking of modality (and, incidentally, of other areas of grammar).
The hierarchy of irrealis modality allows us to make some predictions about the types of modality which will be marked by the irrealis.
The next stages of the project are: to look at irrealis marking outside modality (in subordinate clauses, in interrogative and negated clauses, etc) and to look at languages where there is more than one irrealis marker.