The name Irreantum is the first word written at the top of page 33 of 𝓞. After writing it, Oliver Cowdery immediately corrected the spelling by supralinearly inserting a second a letter after the first one, thus Irreaantum. Later, however, with very light ink flow, this second (supralinear) a letter was crossed out. There is no strong motivation for correcting the spelling the second time except that the additional a must have been an error that Joseph Smith and Oliver Cowdery later caught, possibly when Oliver read back the sentence to Joseph and the spelling was rechecked. One possible explanation for Oliver writing down the extra a in the first place is that he may have momentarily thought he had missed the single a when he first wrote down the name (especially since he would have just turned to a new page of 𝓞, which could have distracted him). Eventually, Oliver realized he hadn’t missed the a, so he crossed out the extra one.
Less plausible is the suggestion that Oliver just thought the double a spelling was somehow incorrect and made the last spelling change on his own. Since Joseph Smith was controlling for the spelling of Book of Mormon names (by spelling them out letter for letter, when needed), one would think that in taking down the dictated text, Oliver would do his best to write down the correct spelling as Joseph gave it to him. Thus the final spelling, Irreantum, is most probably the correct spelling.
The name Irreantum appears only here in 1 Nephi 17:5. This spelling with just one a is consistent with the spelling of other Book of Mormon names and words. Consider, for instance, the following spellings: Archeantus, Hearthom, neas, Seantum, and Teancum. Each of these has ea, not eaa. And in two cases, the ea is followed by nt (just like Irreantum). In addition, there are two names that end in -iantum, Coriantum and Moriantum. Other similar names with ia followed by an n and a consonant (most often t) include Anianti, Corianton, Coriantor, Coriantumr, Gadiandi, Gaddianton, Giddianhi, and Moriancumer. The only Book of Mormon name with a long a is Paanchi, but here the aa is preceded by a consonant rather than a vowel.
Summary: Maintain the spelling Irreantum rather than Oliver Cowdery’s initial correction to Irreaantum; even though the second a letter is later crossed out with weaker ink flow, the resulting spelling Irreantum does appear to be the correct one.