Answer:
Unlike most languages, Interlingua does not contain irregularities. "Irregularity" is sometimes confused with "plurality" or "variety." For example, an Interlingua noun can be formed from a verb with either -mente or -tion. Many other languages also have both of these endings. Their availability is not considered an irregularity in Interlingua or in any natural language. In fact, they are considered regular in Interlingua because they accurately reflect the natural languages.
For the natural languages, an irregularity is generally defined as a word or grammatical element that does not follow the usual linguistic patterns. More specifically, a grammatical irregularity is a word that does not follow the usual pattern of inflection when forming, say, verbs tenses or noun plurals. In English, an irregular verb is one that does not form its past tense by adding -ed, -d, or occasionally -t, but through a stem change. For example, the verbs swim, teach, and wear are considered irregular because the past tense forms swam, taught, and wore are formed with a stem change. By contrast, satisfy, score, dream, and pat are all considered regular. The past tense forms satisfied, scored, dreamed or dreamt, and patted are all formed by adding -ed, -d, or -t, even though they display some variety.
For Interlingua, an irregularity would be a word or grammatical element that does not follow the linguistic patterns of the natural languages. Since all Interlingua words and elements follow those patterns, they are all considered regular. Granted, nearly all Interlingua words and elements go a step further and follow the linguistic patterns of Interlingua itself. Consider grammatical inflections, for example. Verbs in Interlingua never form their past or other tenses by changing their stem; they all use the ending -va. Similarly, all nouns that are formed from verbs use a regular ending, such as -tion or -mente. For Interlingua, however, these words--and the patterns themselves--are considered regular because they accurately reflect the natural languages.
For a few Interlingua words, however, inflections are really beside the point. The most commonly used examples, by far, are the short forms ha, va, and es, meaning has, have; goes, go; and is, am, are. These words do not form other words at all. They are simply used by themselves as short, present tense forms. They are always the same:
io ha: I have
tu ha: you have
ille, illa, illos ha: he, she, it has
nos ha: we have
vos ha: you have (plural and formal)
illes, illas, illos ha: they have
These words are considered regular for the same reason as the words and rules described above: they are taken regularly from the natural languages. Gopsill shows why Interlingua allows for some variety, but does not contain irregularities.
References
Gode, Alexander (1951). "Introduction." Interlingua-English Dictionary. New York: Frederick Ungar Publishing Company. Full title: Interlingua-English: A dictionary of the international language. Second edition: 1971.
Gopsill, F. P. (1995). "Es interlingua vermente perfecte? On dice, que illo es plen de irregularitates." In Prejudicio, ignorantia, negligentia: Interlingua in question e responsa. Sheffield, England: British Interlingua Society.
O'Dwyer, Bernard (2000). Modern English structures: Form, function, and position. Peterborough, Ontario, Canada: Broadview Press, Ltd.