Two of Bank of Ireland's largest shareholders, Wilbur Ross and Fairfax Financial, began selling a 6.4 percent share in the country's largest lender on Tuesday, the placing's bookrunner Deutsche Bank said.
Billionaire investor Ross and Fairfax boss Prem Watsa were among a group of North American investors that kept the bank out of state hands in 2011 when they bought a 35 percent stake only months after Ireland signed up to an EU/IMF bailout.
(Read More: Ireland exits bailout: Mission not quite accomplished)
The group invested in the now 14 percent state-owned bank when its share price was at about 10 cents. The shares have risen significantly since then, jumping 120 percent last year and hitting a high of 0.39 euros this year.
The bank's shares were down 6 percent at 0.34 euros by 09:10 London time on Tuesday, having slipped 7 percent on Monday after publication of its full-year results.
(Read More: Ireland's bailout exit feted by bond markets)
Ross owned more than 2.9 billion Bank of Ireland shares, or 9.1 percent of the bank, before the announcement. Fairfax held 2.8 billion shares, or 8.7 percent.
Deutsche Bank said the accelerated bookbuilding of the 2.1 billion euros of shares on offer was open to institutional investors only.
Follow us on Twitter: @CNBCWorld