Rucio UI creates metalinks for datasets with a maximum of 500 files files within the dataset (Download icon next to this info button). Triggering the download will give you .meta4 file that can be opened with a suitable metalink downloader (or browser plugin) like DownThemAll!. To open the metalink file with DownThemAll! follow this guideline.
Note: If there are no available RSEs that store the dataset no download icon will appear.