https://www.youtube.com/watch?v=bXKYdqIUHQI
This is the feat. Spoilers for Game of Thrones, obviously. And yes, just if you're curious this happens in the books as well. Seeing as he crushed the skull (didn't shear it) compressive strength would be used. Seeing as that is literally the amount of pressure you can apply before an object deforms/fractures.
Compressive Strength of Bone - 170 MPa
Weight of the Skull - 997 g
Density of Bone - 1.6 g/cm^3
997/1.6 = 623.125 cm^3
170 MPa*623.125 cm^3 = 105,931 J
Now just to be sure I'll do shear strength too, ok?
Shear Strength of Bone - 51.6 MPa
56.1 MPa*623.125 cm^3 = 34,960 J
Results
Head Crush (Compressive) - 105,931 Joules, Wall level
Head Crush (Shear) - 34,960 Joules, Wall level
Sources
Compressive Strength - https://en.wikipedia.org/wiki/Bone
Weight - http://www.answers.com/Q/What_is_the_average_weight_of_a_human_skull?#slide=2 (not the best I know but it's the best I found)
Density of Bone - http://hypertextbook.com/facts/2002/AnnaYarusskaya.shtml