Initial High Front Vowels or Where the j in eòrna comes from

Actually this isn't so much about where it comes from but rather how you can tell when it's there and where it isn't.

In a nutshell, it appears when you have a word with an initial high front vowel followed by a back vowel. Which means what exactly?

High front vowels are [i] and [e] in Gaelic, back vowels are [a] [o] [ɤ] [ɔ] [u] and [ɯ]. So if you get any combination of those two (in the said order), you get and initial glide, the [j]. Don't confuse this with the [ʝ] sound (slender gh and dh) - [j] has much less friction. This is a sound you actually get in English words - year, yarn, yen etc.

A few examples:

eòrna	[jɔːRNə]
iolaire	[juLɪrʲə]
Eòghann	[jɔː.əN]
iubhar	[ju.əɾ]
eallach	[jaLəx]
eòin	[jɔː.Nʲ]
ionnsaich	[jũːNsɪç]
earrach	[jaRəx]
eòlas	[jɔːLəs]

Unfortunately, this rule doesn't always work and you get words like iorram [iRəm]. The short answer to this problem is that it has something to do with Old Irish. So the best way of dealing with this issue is to expect a [j] glide when you get e or i before a back vowel, especially if the first syllable contains a long vowel and learn when the exceptions apply.