The instrument used to measure the volume of an irregular object is a graduated cylinder (or sometimes a beaker) and water. Here's how it works:

1. Fill the graduated cylinder with water: Note the initial water level.

2. Submerge the object: Carefully lower the irregular object into the graduated cylinder. Make sure it's fully submerged and doesn't touch the sides.

3. Observe the new water level: The water level will rise. Note the new water level.

4. Calculate the difference: Subtract the initial water level from the final water level. This difference represents the volume of the irregular object.

Why this works: The object displaces a volume of water equal to its own volume. This is based on Archimedes' principle.