If the Pokemon that Focus Sash is attached to is attacked with an attack such as Cobalion EX's "Steel Bullet" that ignores effects on the Defending Pokemon, is it Knocked Out?
No. The full damage would be done first, and then afterwards the effect of Focus Sash would reduce the damage counters to leave 10 HP remaining.
Source: Furious Fists FAQ (2014-08-28), TPCi Rules Team (2014-08-28)

Relevant cards

Note: All applicable cards may not be listed below.
XY3_91