Note that there already is a tax that people are paying 'where the customers are'. It's the VAT, which is one of the biggest sources of income for a state.

To complicate the situation, a French citizen, living in Spain, the sole owner of a company incorporated in Ireland, buys software from an American company to run for a customer in Brazil.

Which country/countries should be able to tax the revenue?

The company would pay corporation tax in Ireland. The French citizen, if resident in Spain, would probably pay taxes in Spain (generally countries tax residents).

The customer may need to pay sales tax on the purchase to Brazil, if Brazil has such a thing.

But I agree with your point - it does get quite complicated and bureaucratic.

> The company would pay corporation tax in Ireland.

You'd assume that, but you could also very well be wrong. Depending on tax treaties and the "effective place of management" principle, he could be liable for corporation tax in Spain or in both Spain and Ireland.

One reason why tax laws get very complicated very fast is because every country wants to tax everything it can which inevitably means that two different countries end up taxing the same thing. To avoid this, you get double taxation treaties and loopholes.

In the example above the American company presumably has most of its employees in the US and ought to pay significant tax there somehow.

It will: the process of paying employees is heavily taxed. Who writes the check and what the nominal salary is doesn't matter too much... between the cost to the company, and the employee getting to spend the money, the government gets what 30-40%? That's a lot. And it's hard to avoid, you can't move a hundred engineers to the Bahamas at the stroke of a pen.

It gets really hairy in cases like Google and Facebook, where the guy paying the service might be a US agency paying to show ads to YouTube/FB users in Europe.