Hi @yesh,
Yes it is possible for QuickSight to access and decypt data. If you are using KMS to encrypt your data you need to give the IAM Quicksight role access to the KMS key to use and decypt data using this key.
For the second part of your question, yes you can work with folders within QuickSight to restrict access to QuickSight to certain users and groups. We recently launched restricted folders which can provide additional guardrails when it comes to the sharing and management of assets within these folders.
I am marking this reply as, “Solution,” but let me know if you would like additional information on either of the above points.