From A Wiki of Ice and Fire
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.
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. Stonesnake and Jon Snow scale the Skirling Pass at night to ambush a trio of wildling watchers, where they kill two of the sentries. 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 confirmed survivor while Stonesnake is ordered by Qhorin Halfhand to climb through the Frostfangs on foot to warn the Night's Watch of the wildling invasion.
A Storm of Swords
Stonesnake is lost in the Skirling Pass. Several wildings attempt to follow the ranger up the mountain, but cease pursuit once the terrain becomes too rough for them and their horses to follow him.
Chapters that takes place at the Skirling Pass