EASA learning objective 022.05.02.01.01 says: 'State that during the alignment process, the inertial platform is levelled (INS) or the local vertical is determined (IRS), and true north/aircraft heading is established.'

The initial alignment of the IRS needs to be carried out when the aircraft is stationary and on the ground. A set of three linear accelerometers, one for each aircraft axis, will sense gravitational acceleration that will enable calculation of the local vertical. Although an IRS has no physical platform it uses the local vertical to create a mathematical one to enable the accurate measurement of aircraft acceleration.

3 ring laser gyros detect angular rotations around the aircraft axes. As the aircraft is stationary during alignment any rotation must be due to Earth rotation, this input is used to calculate latitude and the direction of True North

Although latitude can be calculated by the IRS its accuracy is not sufficiently precise, so a manual position input is required from the flight crew for the alignment sequence. The IRS will compare the inserted latitude with the calculated one to make sure that they agree within certain limits as part of its alignment performance check .

The IRS cannot calculate/measure longitude, therefore the only reference is the one provided by the pilot during the alignment process.