Sure... C is basically C... but then there is the SPECIFIC stuff that Arduino hides from you... that this code doesn't. You kind of have to figure out how to make the source code you have make a compromise with what the Arduino environment wants.
Code written for the AVRGCC environment might "almost" be a simple as cut and paste, however, code written for IAR's version of C may not be so easy. If I recall correctly, IAR might do some heavy lifting that isn't obvious.
IE;
#define __IAR_SYSTEMS_ASM__ <-- This can hurt! what does it do?
These two you have... but are they compatible?
#include <io4414.h>
#include "ina90.h"
The stuff below definitely needs fixing:
#define Xtal 8000000 // system clock frequency
#define prescaler 1 // timer1 prescaler
#define N_samples 128 // Number of samples in lookup table
#define Fck Xtal/prescaler // Timer1 working frequency
if you don't get the clock stuff worked out... you will not get the frequencies you want.
You kind of gave yourself a big conversion task ahead so you will need to know what you are trying to do.