From what I understand, you can't fly as a required flight crewmember (pilot) on a JAA airplane unless you have a JAA licence or a JAA validation based on your ICAO licence.
To validate your ICAO PPL/IR, here's what the JAR-FCL says...
Appendix 1 to JAR–FCL 1.015
Minimum requirements for the validation of pilot licences of non-JAA States
A private pilot licence with instrument rating issued in accordance with ICAO Annex 1 by a non-JAA State may be validated subject to conditions by a JAA Member State in order to permit flights (other than flight instruction) in aeroplanes registered in that JAA Member State.
To validate such licences, the holder shall:
(a) complete, as a skill test, the type/class and instrument rating of Appendix 1 and 2 to JAR-FCL 1.210 and Appendix 3 to JAR-FCL 1.240;
(b) demonstrate to the satisfaction of the Authority in accordance with Subpart J, that a knowledge of Air Law and the Aeronautical Weather codes, as well as the Flight Planning & Performance (IR), Human Performance in accordance with Appendix 1 to JAR-FCL 1.470 has been acquired;
(c) demonstrate a knowledge of English in accordance with JAR-FCL 1.200;
(d) hold at least a valid JAR-FCL Class 2 medical certificate including hearing requirements in accordance with JAR-FCL 3.355(b);
(e) hold R/T privileges acceptable to the Authority, (f) comply with the experience requirements of 100 hrs PIC instrument flight time
To convert your ICAO PPL, here's what the JAR-FCL says...
Appendix 2 to JAR-FCL 1.015
Conversion of a PPL issued by a non-JAA Member State to a JAR-FCL PPL
The minimum requirements for the conversion of a private pilot licence issued by a non-JAA Member State to a JAR-FCL licence are:
(a) the applicant shall hold a licence issued in accordance with ICAO Annex 1
(b) the applicant shall hold at least a JAR-FCL Class 2 medical certificate
(c) to hold R/T privileges acceptable to the Authority
(d) the applicant shall comply with the following flying experience requirements : ›100 hours as pilot of aeroplanes.
Any further JAR-FCL requirements :
(1) Pass a written examination in Air Law and Human Performance and Limitations
(2) Pass the PPL skill test as set out in Appendix 1 to JAR-FCL 1.130 and 1.135 and Appendix 2 to JAR-FCL 1.135
(3) Fulfil the relevant requirements of Subpart F