Anglo-Saxon (Old English) was very phonetic too but it is clear that over time certain words ended up diverging between dialects and in the interim scribes were sometimes confused as to which letter to use for the intermediate sound.

Slender ch, f, and th (whether it be /h/ and /θ/ - and it has been both in the course of Irish history) aren't all that different from each other. In many languages that have the ch sound, children go through a phase where they confuse the phonemes until they become sufficiently aware to differentiate them.

The examples I gave above also show that they are related sounds even today.

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

it was θ in Old Irish



the Irish scribes weren't children



chéin is a lenited form of céin. And there's no other example of f alternating with c, even in the modern language. The sounds /k'/ and /f'/ have nothing in common, except that they are slender. They aren't even pronounced in the same place in the mouth...
Why would a scribe write "céin" if he says /f'e:n'/ and why would he write féin if he says /k'e:n'/ ? That seems strange.

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

I wasn't comparing céin and féin, I was comparing chéin and féin, which certainly _are_ similar in light of cluiche/cluife.

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

chéin is the lenited form of céin, right?

_________________
Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne