Is Sheela na gig a girl or an ability? Or do you mean the update to the 2nd row mythic girls?
Sheela na gig is the mythic girl from the Country cousins set (green / earth set that is currently 5th from the bottom of collection list). If you click on the girl, then Ability at bottom of list (at the right), you can see the cost to upgrade their ability (called Venom for Sheela).
To upgrade to ability level 2, you would need to level up the girl to level 5, and have 5 libido stones of the correct colour / element.