"Is it really necessary for it to go all the way to Limerick?"

I suppose you're implying that Shannon Airport might was well be the southern terminus of the route? I would think not.

For international travelers, it's probaly true that many trips will originate from, and return to, the airport, and that most such trips will head north, to one or another of the many possible dstinations in that direction.

But surely there would be significant numbers traveling between the airport and the nearest large city, Limerick, wouldn't there?

And as far as local/domestic travellers are concerned, there is certainly a much larger population surrounding the Limerick station than there is out around the airport...