Well she is another DPS burst Champion, but it depends upon the user of the Champion itself.
Most of the time when an emeny Irelia uses her ultimate, she often misses the first 3 shots, unless your running away.
So really, all you need is a blind and Thornmail and you can almost counter everything. Her strike heal is just a WW rip off, so really, she's UP to an extent, but that blink is crazy