It never was.
Donegal never had even a kilometre of mainline or standard gauge track at any time, even at the heigh of the expansion of the Irish rail system. Kerry, by contrast, had extensive standard gauge mainline tracks from the late nineteenth century.
We'll it was vaguely comparable before any train lines were built.
My point was simpler. At this point in time Kerry has existing infrastructure Donegal has none. No point comparing them now.
https://donegalrailway.com/railways-of-donegal/