Looking into this a little more: the Marsaglia RNG that mIRC currently uses is quite old and I can find no reviews for the variant being used. So I have decided to change the RNG algorithm to the 64bit version of Jenkins's Small PRNG
. This RNG is estabilished and well-reviewed, the code is short and simple, and it passes a wide range of tests eg. PractRand. This will be in the next version.