A message on the page says "This is a high level category - please do not add individual profiles."
I do not understand why profiles would not be added to the Category:County_Cavan which currently has 88 profiles. The help page https://www.wikitree.com/wiki/Help:Category_Names
states "Ideally, bottom-level categories should be much smaller, with no more than a few thousand people in them."
Current hierarchy is 1- Regions, 2 - Europe (en), 3- Ireland, 4 - County Cavan is at the 4th level, which seems a mid-level rather than a high level.
A fundamental research for sources in Ireland is to look at the locality which so many times is simply the county name. I hope wiki-tree categorization for Ireland will allow profiles to be attached to the County category.