LPC1347 group level interrupt clearing
change interrupt type to low level sensitive 


Level interrupts are always (re-)triggered when the pin is in the selected state - low in your case. You can either mask the interrupt bit or change the level to high (or to rising edge) in the interrupt handler. 

So in a nutshell, you need to turn off the interrupt source, or the interrupt handler will tail-chained into and thus run in a loop as long as the pin is in the low state.