EI-
The main reason for the use of foreign registers in jurisdictions such as Italy is perfection of security. The Italian register does not permit lessors to reposess their aircraft easily in a bankrupcy because of local judicial procedures. It could take a very long time for a lessor to prove its ownership or financial interest ina n aircraft in the Italian court system. In addition there is no efficient recognition of mortgages. The same is also true in Russia.
Ireland has very favourable double taxation treaties with many countries and this, coupled with gererous tax breaks offered by the Irish government, engouraged leasing companies to locate there - particularly in Shannon and the Dublin Doocklands - both of which were heavily subsidised by the government.
Bermuda and Ireland are the most favoured 'flags of convenience' for commercial aircraft. Corporate aircraft are additionally registered in the Isle of Man, Aruba and Cayman as well as the US.
Hope this helps
WOD-DET-DVR