In relation to authorisations as certifying staff to maintenance technicians who are not EASA Part 66 licensed

As set out in paragraph 145.A.30(j)(1) and (2) and Appendix IV to Part 145, the holder of an ICAO licence shall demonstrate that he has acquired the training on human factors and airworthiness regulations detailed in modules 9 and 10 of Appendix I to Part 66, and the aircraft type training indicated in point 1(e) of Appendix IV, which shall be equivalent to that in Part 66 and for which such a comparison shall be made. 
 
Further information can be found in the following table provided by EASA.

NOTE: after BREXIT Part 66 licences issued by the United Kingdom are considered national licences under 145.A.30(j)1 and 2, and may be used under the conditions of Appendix IV to Part 145. 

Please note that C/S or S/S authorisations issued on the basis of "foreign domestic licences" MAY NOT be used within the territory of the EASA Member State (neither for line operations, nor for base operations, nor for air flight operations (AOG) nor for occasional line maintenance). The only exception to this is 145.A.30(j)(5).