Fortis, based jointly in Belgium and the Netherlands, is among the 20 largest financial institutions in Europe.
It has a market capitalisation of €37 billion, which compares to AIB's €16.5 billion.
The bank has a presence in 44 countries, often through joint venture arrangements. The most relevant of these for An Post is Fortis's decade-long link with the Belgian Post Office.
This relationship began with banking products such as savings accounts and credit cards, more recently progressing into insurance.
Many observers believe that insurance could represent the biggest business opportunity for the proposed An Post relationship.
Fortis is already the largest bancassurer and the leading broker-based insurer in Belgium. It is in the top three in the Netherlands and also offers bancassurance in Spain, Portugal and China.
In the UK, it is the third-largest motor insurer.
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