Does the LPC1343 MCU need an external clock for USB operation?
In general, the IRC oscillator is not precise enough to use for USB. This is why the LPC13xx series has a separate PLL for USB. Quoting document AN11392: Guidelines for full-speed USB on NXP's LPC microcontrollers:

When using a full-speed USB peripheral, it is important to use an external crystal, or a 
tight frequency tolerance ceramic resonator, for the timing element. The internal 12 MHz 
±1 % RC oscillator, while very accurate, does not have the accuracy required for USB. 
The USB peripheral operates at a clock frequency of 48 MHz, so the crystal must be 
chosen such that a multiple of its frequency is equal to 48 MHz. A parallel resonant 
12 MHz crystal with a data signaling tolerance of 0.25 %, or ±2500 ppm is required.