I basically learnt that words that start with a 'm' or 'p' get 'im' as a negative prefix, whilst words starting with 'r' get 'ir' in such a case (irreverent, irrelevant).
However, I stumbled upon 'realistic'. I'm almost sure it is 'unrealistic'. A quick search on Google revealed 13 million hits for 'unrealistic'; 'irrealistic' on the contrary only results in 14.000 hits.
Therefore, is 'unrealistic' an exception to this rule?