It depends on the accuracy of the needed 1.5V and in the possible variations of your 2.5 power supply. If the 2.5V supply is stable, you can just use a divider (transistor made or resistive) to achieve the 1.5V and to buffer that voltage to be able to source or sink some current.
If the 2.5V is not very stable, you must first generate a reference voltage and build from it your 1.5V with a regulator architecture.