The particle
a often disappears in speech. See eg. in
corkirish blog which states “The important thing is the lenition, not the particle itself, which is often only theoretically there” (because it’s often inaudible in fluent speech).
So might be it’s written by somebody using an
eye dialect spelling – spelling in a non-standard way to draw more attention to actual pronunciation. But it’s weird to see something like that in learning-related…
You’d rather see that in potery, insongs lyrics, and most often after vowels, the vocative particle sometimes indicated by an apostrophe, eg. in
Órú Mháire Bhruinneall, bhláth na Finne or
’Mháire Bhruinneall, ’bhláth na Finne (but also
a Mháire Bhruinneall, a bhláth na Finne); or in
Go mbristear do chosa, ’bhean Pháidín (more often as
Go mbristear do chosa, a bhean Pháidín); or
Téir abhaile riú, Mhéaraí/Mhary/Mháire.
Similarly some people seem to write things like
bhfuil ’s ag éinne…? instead of
an bhfuil a fhios ag éinne…? without the interrogative
an but with eclipsis.