And Cleasby and Vigfusson are authoritative on the Old Norse meaning, suggesting that this is indeed the most likely etymology
cleasby-vigfusson-dictionary.vercel.app/word/i-spen

Seems like MacBain might have been a source of information here, at least regarding the Scottish Gaelic cognate
en.wikisource.org/wiki/An_Etym...

This 2014 paper is likely the uncredited source of information for the 2021 Wiktionary edit. It's the only published research I could find on this word's etymology. The information seems reasonable enough, though no historical form is yet attested. But we have the Scottish Gaelic cognate “isbean”.

Congratulations!

I noticed that in the interpretive editions of inscriptions, where certain interpreted markings seem to be using digital characters that are not currently rendering for me on my phone, such as the i in maqi, and the c in mucoi, in the screenshot example.

Hugh MacCurtin's 1728 "The Elements of the Irish Language"

There's also an “á bé cé dé...” system. The earliest reference to this system I could find is the 25th edition (1890) of "An Cheud leabhar Gaedhilge - First Irish book". Perhaps earlier editions had it too, but this is the earliest I could see.
archive.org/details/cheu...

Interesting point in the latter half of this paragraph on Classical pronunciation, giving more insight into the nature of lengthening/diphthongisation of short vowels in Munster Irish when preceding fortis sonorants only when the sonorant falls in the coda position of the syllable.