Sean Quinn was once the richest man in Ireland. In 2008, Forbes estimated Quinn's riches reached $6 billion. Today, however, he owes Anglo Irish Bank $2.7 billion and in November, he claimed to only have £11,000 in the bank. In January, an Irish court declared him bankrupt.

Quinn is the epitome of the self-made businessman. At age 14, he dropped out of school -- unable to read or write. But with a loan of just £100, he began building a business -- which came to be known as the QUINN Group -- that started with the simple sale of gravel from his family farm to local builders.

The QUINN Group quickly ventured into other building materials and business grew throughout Europe. But Quinn couldn't escape Ireland's 2009 property collapse, which crippled the country’s economy and destroyed his fortune, and the bank accounts of thousands of Irish citizens.

The extent of his folly is still being discovered, but we can learn four important investing lessons from what we know so far:

1. Don't Forget to Diversify.

Quinn didn't get rich through banking, but he can trace his own fall to that of one bank. While the QUINN Group invested in insurance, an international hotel chain, a glass company and a cement company, the tycoon continually increased his shares in a single bank, Anglo Irish Bank. He couldn't recover when the bottom fell out.

Lesson Learned: Do a careful review of your investments. Are you so deeply invested in one area that if they went south, it would destroy your portfolio? If so, no matter how great the investment seems, diversify. If this folly can bankrupt a billionaire, it can bankrupt you.

2. Don't Get Greedy.

Quinn admitted to the media that his hefty stake in Anglo Irish Bank was 'greedy.' His bankruptcy could bar him from business activities for 12 years, force him to liquidate possessions to pay his debts and leave him on a shoestring budget. That should be quite a jolt for a guy who has vacation property in Ukraine, Russia and India.

Lesson Learned: Be careful not to become swept up in your own quest for wealth. Money can be highly motivating, but when it becomes an obsession, the results can be disastrous.

3.Transparency Is The Best Policy.

What really sunk Quinn was his huge investment in Anglo Irish Bank, which was once the largest lender for Ireland's now devastated construction industry. He was able to obtain a 28% stake in the company because the Irish law allowed the QUINN Group to hide the scale of its investment from other shareholders.

Lesson Learned: This part isn't entirely on Quinn. The market regulations didn't require him to reveal his huge investment to other stockholders. If other shareholders knew of his hefty stake, perhaps there would have been enough backlash to push Quinn to sell some of his shares.

4. Be Careful When You Cross Borders.

Quinn attempted to have his bankruptcy filed in Northern Ireland, which is part of the United Kingdom. While he is from Northern Ireland and his business originated there, the QUINN Group actually operates in the Republic of Ireland. The catch? Bankruptcy requirements in the Republic of Ireland are much stricter than those in Northern Ireland.

Unfortunately for Quinn, the courts caught on and forced him to file in the Republic of Ireland.

Lesson Learned: International markets offer great opportunities and advantages to make money. However, be sure you know all the implications of doing international business. Quinn only thought of the tax benefits of where he headquartered his business, never thinking of the implications that location would have if he ever needed to file bankruptcy.

The Investing Answer: Usually, following the moves of a billionaire is sage advice for any investor, but the story of Sean Quinn teaches some valuable lessons:

  • Regularly review your portfolio and stay diversified.
  • Don't let greed overcome your common sense, especially when it comes to dangerous shortcuts.
  • Always research international investment ideas thoroughly.