The number that is irrational among the options given is the square root of 23.
- 23.85 is a decimal (and can be expressed as a fraction).
- The square root of 49 is 7, which is a rational number.
- The square root of 23 is not a perfect square and cannot be expressed as a fraction, making it irrational.
- 1/2 is a rational number.
Therefore, the answer is the square root of 23.