Bank of Ireland made a pre-tax profit of €399m (£259m, $437m) over the first six months of 2014, the first time the bank has been in the black since the economic crash of 2008.
It represents a huge turnaround from the €500m loss incurred over the same period of last year, which saw the bank coming off a period of restructuring.
The results have been attributed to the uptick in the Irish economy, with state-owned Allied Irish Banks (AIB) also announcing a return to profit earlier this week. Ulster Bank, which is owned by the Royal Bank of Scotland, turned half-year profits of £69m in the Republic of Ireland.
The Irish government still retains a 14% stake in Bank of Ireland, but it was the only major bank to avoid full nationalisation during the crippling financial crisis.
The performance mirrors that of Bank of Ireland UK, operational in Northern Ireland, which made a £46m profit, up from a loss of £97m in the first half of 2013.
The bank's CEO Richie Boucher said: "We have continued to make good progress against our strategic objectives in the first half of 2014. We are profitable and generating capital.
"In Ireland, we continue to support and benefit from the accelerating economic recovery; our new lending in 2014 makes us the largest lender to the Irish economy.
"The favourable economic outlook and the strength of and momentum in our Irish and international businesses gives us confidence in our ability to deliver attractive and sustainable returns for our shareholders."
The banking sector in Ireland has been downsized significantly since the crash, with many international lenders either scaling back on their Irish operations or abandoning them completely.
It's left a streamlined sector with fewer lenders in every space. The return of the three major banks to profit, then, will be welcomed by the Irish government.