Shares opened at 4.76 euros in Dublin, above the initial public offering (IPO) price of 4.40 euros. Shares also began trading in London where the deal was the biggest IPO by market capitalisation in almost six years.

The size of the IPO will rise to 28.75 percent if demand proves strong following the debut, after the government included a greenshoe, or over-allotment, option. The increase would add another 400 million euros to state coffers.

(Reporting by Padraic Halpin; Editing by Mark Potter)