Blackstone manages some $150bn (€114bn) of assets. It owns major stakes in soft drink maker Orangina, Danish telecom provider TDC and United Biscuits, and soon Eircom.
It's believed to control the biggest property portfolio in the world, plus a plethora of hedge funds.
An "advisory" arm means Blackstone effectively owns its own investment bank.
The company was founded in 1985 by Stephen Schwarzman and business partner Peter Peterson, who had been senior bankers together at Lehman Brothers.
They started Blackstone with a $400,000 investment but the business grew rapidly hoovering up money from blue chip US investors -- including many of the top "Ivy League" American universities -- to buy and sell companies, property and securities.
It remained low profile until a stock market flotation in 2007.
Documents released ahead of the share sale revealed the staggering wealth of Mr Schwarzman in particular, and hit the headlines.
The stock exchange listing valued his share of the business at $10bn, and his salary was revealed to have topped $300m the previous year.
He was vilified by unions and lauded as 'America's leading capitalist'.
He courted controversy with a high profile 60th birthday the same year, where he is reported to have paid singer Rod Stewart $1m for a half-hour set.
Mr Schwarzman has a long association with the US Republican Party.
Last year he held a fundraiser in support of Republican Party candidate Mitt Romney, a long-time business associate, at his $30m Manhattan apartment. He is an outspoken critic of US President Barack Obama.
Mr Schwarzman first disclosed his interest in Ireland in 2010, in a speech delivered at a Goldman Sachs event.
The tone was typically unforgiving.
"As we look at the current situation in Europe, we're basically waiting to see how beaten up people's psyches get, and where they're willing to sell assets," he told his fellow Wall Streeters. "You want to wait until there's really blood in the streets."