> Tax avoidance is seeking to minimise a tax bill without deliberate deception (which would be tax evasion) but contrary to the spirit of the law. It therefore involves the exploitation of loopholes and gaps in tax and other legislation in ways not anticipated by the law. Those loopholes may be in domestic tax law alone, but they may also be between domestic tax law and company law or between domestic tax law and accounting regulations, for example. The process can also seek to exploit gaps that exist between domestic tax law and the law of other countries when undertaking international transactions.
(I'm a former tax accountant, at KPMG - these definitions are uncontroversial AFAIK)
It'll be interesting to see what they do with the profits from their companies (both the US and Ireland).
If they keep a chunk of income in Ireland and only transfer it to the US during tax amnesties then that's one standard form of avoidance.
If the US Dropbox entity starts paying a large sum of to Dropbox Ireland in order to license some part of the technology then we're in to the cunning world of tax avoidance proper.