Interesting. I'm guessing that this trend is due to the influence of the English language, there being no nasal vowels in English. The problem is that there are essentially no monoglot Irish speakers left, not to mind monoglot communities, and so the Irish language now exists entirely under the shadow of English. Another development is that Irish broad r is gradually being replaced by the English r sound among native speakers (although slender r remains intact). Even some native speakers on radio or television who otherwise seem to have perfect diction use English r.

Depends on how far into the past you go, and at whose writings you look. Old Irish had generally éo, éu pronounced /eːu̯/ in words like béu, béo ‘alive’, céul, céol ‘music’ which then often was written as eó in Middle Irish (suggesting it turned into /e̯oː/ or just /ʲoː/ like in modern Irish), then in Classical Gaelic often also eó – though of course you can find all kinds of spelling with the fada on either vowel or without any fada since early Middle Ages to the end of the classical era – céol, ceól, ceol – as the manuscripts weren’t very consistent in marking long vowels anyway. Compare eg. Keating’s tuismhidhtheóir and taidhléoir. And modern Scottish Gaelic also writes this long, eg. beò, ceòil (though they have shortened unstressed vowels and got rid of some final ones, so Sc. Gaelic equivalent of Irish both -óir and -aire is just -air).

In Classical Gaelic you also sometimes get long vowel markings in ao(i), like eg. Gáoidhealg, cáora, láoch from OIr. long /aːi̯, oːi̯/ diphthongs in Goídelc, cáera, láech/lóech.

(And the words with short /o/ in eo afaik generally evolved from some other vowels, like seo from OIr. so, se, eochar – not sure what was its original form, but cf. Sc. Gaelic iuchar, leogaim from léigim, also cf. Sc. leigidh, etc.)

Thank you, I see.
The accepted spelling by the time of Dinneen's dictionary was ceol. And I think one of the reasons Ua Laoghaire's books were banned from the curriculum of Irish schools in his lifetime was because he went freelance on Irish spelling, including writing ceól and glanan (instead of glanann), etc. But then his books are invaluable because there is a considerable, but not entirely consistent, attempt to show the pronunciation.