Unless otherwise stated, the Snidhall Isle and An Skellig quests are accessible during your exploration of Skellige in Act I.
The Witcher 3: Wild Hunt eGuide Secondary Quests Secondary Quests: Skellige: Snidhall Isle (1) And An Skellig (2)
Unless otherwise stated, the Snidhall Isle and An Skellig quests are accessible during your exploration of Skellige in Act I.