Kerry FC are "next in line" solely because the FAI has said they are. But not only is there no Kerry FC yet, there's not a "line" either.
In other words, Kerry FC are a "sticking plaster".
"A few years"?
Doesn't sound as if the FAI plans to wait that long:
"Commence consultation on the formation of an Irish football pyramid by Q4, 2022 with an agreed and transformed football pyramid structure in place by 2025"
https://www.fai.ie/domestic/news/fai...-2025-launched
While you've cited four possible entrants who, even if all are willing and successful, would still be somewhat short of a viable third tier of 8 clubs (minimum)
Correction: "If a club progresses
successfully from youth football to LOI etc". That is, if Kerry fails, that puts an end to that experiment.
And besides, apart from Meath, who else have you in mind? Remember, not every new entrant in a 3rd tier is sure to manage, even at that level, never mind progress to the next one. Meaning that the pyramid shouldn't just end at the 3rd tier, as though it was somehow hanging in the air, you need Promotion & Relegation in both directions for it to work.
Except that they haven't gone "from the lowest youth league group all the way up to senior [over] a few years", the club is effectively an entirely new creation, irrespective of where it hopes to draw its players and supporters from.
Anyhow, though I've only been to Kerry a couple of times, it seems a nice place with nice people, so good luck to them.