Please allow JIRA administrators to see private filters and dashboards in the GUI.
Some use cases:

• A user created a filter, forgot to share it, sent the link by mail to some people who should use it, and ran off for holiday. Sure, the people who got the mails will complain. The adminstrator should be able to acces the filter in the GUI and to share it.
• People who leave the company do not delete their private filters and dashboards before they go. Over time, this relicts will pile up. Adminstrators should have the option to clear private filters and dashboards after a user has left.

It's not feasible to query the database to find out which private filters exist and who the owners are, and alter the database.

First, functionalities in the GUI would be more convenient to use.
Second, you cannot assume that JIRA administrators have access to the database. In our company, I have to contact central administration and ask them if they could run queries and clean up which would be completely unnecessary if I had some more options in the GUI.

Regards, Evelin

Workaround

The problem with this workaround is that if you don't know the name of the dashboard / filter it could be hard to identify. 

A site admin can go to the Manage filters / Manage Dashboards  system page and take ownership of any filter or dashboard:

Filters: <your site name>.atlassian.net/secure/admin/filters/ViewSharedFilters.jspa
Dashboards: <your site name>.atlassian.net/secure/admin/dashboards/ViewSharedDashboards.jspa

Once that's done they can manage the filter and its subscriptions themselves.

Workaround for identifying filter for which you only have the ID, not the name
Site admins can identify the filter with the following steps. This is currently not possible for dashboards.

1. Go to <your site name>.atlassian.net/rest/api/3/filter/]<FILTER ID>?overrideSharePermissions=true
   Because you are a site admin you will be able to see the name of the filter on that page:
   
2.  You can then go to your filter systems page, search for it by name and change the owner:
   <your site name>.atlassian.net/secure/admin/filters/ViewSharedFilters.jspa