18th century, the grammar by Aodh Buidhe Mac Cruitín (
The Elements of the Irish Language) – an educated native speaker, familiar with the bardic tradition:
Quote:
Tuig faḋeoiḋ an̄ so, u[air?] gur ab ríaġla dęrḃṫa suimęmhla na trí ċre[…?] (go hairiġṫe cré na neasbal) naċ iád na poinge speisialta (…)

then in the manuscript containing the
Irish Grammatical Tracts I (the “general”, “first” bardic grammatical tract,
MS 24 P 8, page 4) – 17th century copy of an older (15th–16th century?) text, written and then copied by very highly educated native speakers – long before Irish speakers thought about writing Irish using the Roman script:
Quote:
(…) seiṁiuġ[aḋ] d[’]oġam[.]
⁊ cía an con̄suine do[-]nī ceiṫre huirrḋiġṫe (…)

Peadar Ua Laoghaire didn’t exclusively own Irish language, there were native speakers before him and after him, there was a literary tradition
long before him. And there even are editions of his own books, authorized by him, using this convention.
So saying that “authentically” it should be “
a h-aon ....go h-Éirinn in the Gaelic script. Before the Roman script was imposed, hyphens were used in such phrases” is plainly wrong. As I wrote,
both conventions coexisted for a long time, and writing
h together with the following word is very authentic for Irish seanachló (as authentic as using the hyphen, and perhaps, historically, even more).