I used to use Kilcoole station on a regular basis to travel to and from Wicklow on the late morning train that ran to Arklow or Gorey. At that time (about three years ago), a basic shelter with a seat was provided on the right hand side just beyond the swing gate that allows access to the platform i.e. adjacent to the portacabin type structure to which you refer. For reasons unknown to me this basic shelter was removed so now, as you point out, there is no shelter for passengers or even a seat. I have never seen the portacabin type structure to which you refer open and to the best of my knowledge, it has never been available for use by waiting passengers.
Alas, it is not now possible to catch a southbound train at Kilcoole and then return there later in the day. Thus a poor service has become no service at all!