Can you please post any of interrupt program using lpc2129
C:\Keil474\ARM\Boards\Keil\MCB2100\BlinkyIRQ\Time.c
http://www.keil.com/download/docs/232.asp www.nxp.com/.../AN10381.pdf
I don't use NXP/LPC processors, but there's surely got to be some code in the Boards/Examples directories that'll get you 99% the way there.
Review thoroughly your chip and processor documentation. Understand interrupt vectoring and interrupt controllers on your chosen part. Review the Keil documentation and examples.
Ask you teachers/colleagues for examples.