Bank of Ireland has announced a €36m investment over five years at its College Green building in Dublin city centre.
Bank of Ireland said the investment is its biggest spend on the building in more than 200 years.
As well as extensive repairs, upgrading and restoration work, the planned €36m includes improved facilities for customers and workspaces for staff.
The project will involve repairs and upgrades to 280 windows, 45 staircases and 20 kilometres of electrical cabling.
College Green also has 54 roofs, 80 roof lights and a combined 2.5 kilometres of roof walkways.
The former parliament building served as the Bank of Ireland head office until the 1970s.
It is still used as a working branch and also provides office space for bank staff in personal, business and corporate lending, and a wide range of support functions.
In 2018, Bank of Ireland added a cultural and heritage centre which is now home to a National Library of Ireland exhibition celebrating the work of Nobel Prize laureate Seamus Heaney.
Begun in 1728, College Green was the first permanent two-chamber parliament building in the world and served as the Irish parliament until 1800.
Bank of Ireland bought the building in 1803 and opened it to the public as a banking hall in 1808.
College Green has served as a bank ever since, making it the one of the oldest banking halls in continuous use anywhere in the world.
Some of the building's windows date from the original 18th century building, and subject to receiving planning permission, these will be removed, restored and reinstalled.
Two 300 year-old tapestries hanging in the former House of Lords chamber will also be cleaned and restored as part of the project.
About 1,500 fire alarm devices and emergency lights will also be upgraded as part of the works, which also aim to reduce the building's carbon footprint by switching as much as possible of its heating systems to run on fully renewable electricity.
The project will be the biggest since the bank undertook a major repair and cleaning of College Green's exterior stonework and facades in 1971.
The project is subject to planning permission and Bank of Ireland said it intends to submit a detailed application to Dublin City Council later this month.
Subject to approval being granted, it will take about five years to complete.
Susan Russell, Director of Retail Ireland at Bank of Ireland, said that College Green occupies a very special place in Ireland's - and Bank of Ireland's - history and heritage.
"We see this investment as a statement of commitment and intent - commitment to our role as custodian of these iconic buildings for future generations, and intent to continue to play a central role in Dublin's future development," Ms Russell added.