From A Wiki of Ice and Fire
The Skirling Pass is a path into the Frostfangs, located beyond the Wall. The pass is named for the keening sound the wind makes as it blows through it.
The pass is a long, twisting course between mountain peaks and hidden valleys. There is no grass save a few weeds and pale lichen clinging to life amongst cracks in the rock. The highest point of the pass is marked with a a stone archway. From the top there is a hidden valley, long and v-shaped, amongst the mountains. A glacier of ice several thousand feet high plugs one end, squeezed between the mountains. Under that icy height is a great lake, its waters a deep cobalt that reflect the snow-capped peaks that surround it. Shadowcats dwell in the Frostfangs.
Other paths into the Frostfangs include the Giant's Stair, if it is clear. 
A Clash of Kings
The Skirling Pass is considered by the Night's Watch as a probable place to find Mance Rayder's wildlings. Qhorin Halfhand's group is sent to scout it for signs of wildlings. Jon Snow captures Ygritte, but releases the wildling rather than execute her. While retreating back to the Fist of the First Men, the scouts are overtaken by Rattleshirt's wildlings, with Jon the only survivor.
Chapters that takes place at the Skirling Pass
References and Notes