"Even if it were not found illegal under EU law (as I've been predicting it would be) all Google would need to do is appoint an agent in Rome, give him a 0.5% margin on all sales and still send all the cash to Ireland."
The way these avoidance scams work at the moment is that an Italian company, wanting to advertise on Italian websites go speak to an agent of Google in Italy. That agent negotiates prices etc and then, for no logical reason, the "sale" is completed in Ireland by someone the Italian company have probably never dealt with in any way shape or form, and Google get to pay Irish tax only.
What this law is, rather clumsily, attempting to do is force that final step to count as a sale in Italy - which 99% of people would probably agree it should be - and thus be subject to Italian taxation. This way big multinationals have to compete on a level playing field with smaller local companies who aren't in a position to play the system in the same way.
This particular implementation may be wrong and almost certainly tramples over a bunch of EU rules, but it's hard to disagree with the principles behind it, that companies should be subject to the tax laws in the countries they do business and not be able to simply divert profits to anywhere they like without consequence.