If you have followed and passed recognized external training for ISO 9001 , you should also have followed ISO 19011 ( common guidelines for auditor to audit quality, environmenta and safety & health systems). Consider to mantain updated the competences of the auditors.
However, it is clear that the internal auditosr should follow a trainining about ISO 14001 clauses to get familiar with the standards. To get familiar with the standard, the training is the most common acvity to follow. ISO 14001 standard have different contents form ISO 9001 .