How do you calculate the ir1 and ir2 in a parallel circuit?

To calculate the currents ir1 and ir2 in a parallel circuit of two resistors r1 and r2, consider the voltage across them. By Kirchoff's voltage law, the voltage across each resistor will be the same, so simply use Ohm's law to divide voltage by each resistor to get each resistor's current.

ir1 = v / r1

ir2 = v / r2

If you don't know voltage, but you do know total current, then determine the total resistance as RT = R1R2/(R1+R2), calculate voltage as ITRT and proceed from there.