Bank of Ireland is facing a €60m-plus claim to compensate British and Irish investors who bought buy-to-let properties.
The bank changed the margin on their tracker mortgages, in a move that more than doubled repayments in many cases.
A campaigner who won a similar case against West Bromwich Building Society last week says he has queen's counsel opinion that Bank of Ireland is in breach of contact.
Mark Alexander, who spearheaded a campaign that led to last week's win in Britain's Court of Appeal, said his group had now raised half of the funds it needs to take on Bank of Ireland in the British courts. Some 13,000 buy-to-let investors, many of them Irish, are affected.
They took out mortgages that were supposed to track the Bank of England rate at a set margin over the lifetime of the loan, but from May 2013 the bank moved the rate from the base rate plus 0.75pc to the base rate plus 4.49pc.
The mortgages were taken out with Bristol & West, which was owned by Bank of Ireland, but has since been shut down by the bank, and the mortgages serviced by Bank of Ireland UK.
Consumer group Which? had criticised the change at the time as "wholly unfair".
One Irish investor who took out a Bristol & West mortgage on a Swansea property said the change meant his interest rate jumped from 0.75pc to 4.99pc, almost doubling his repayments.
Mr Alexander claimed the Bank of Ireland actions were similar to those in the West Brom case.
He founded the website Property 118.com to fight tracker cases.
Mr Alexander said: "We have raised £32,000 [€40,500] through crowd-funding for a variety of cases. We have already made significant progress on the Bank of Ireland case."
He has opinions from two barristers and a queen's counsel that indicated that Bank of Ireland breached the mortgage holders' contracts when it hiked the tracker margin.
Bank of Ireland said the West Brom situation was not comparable to that of Bank of Ireland UK.
But Mr Alexander said his legal advisers were about to issue a pre-action protocol letter as a first step in the legal proceedings "to alert Bank of Ireland that we intend to take legal action".
He said that although the mortgages contracts of West Brom Building Society and Bank of Ireland were different, there were huge similarities.
West Brom has to refund £27.5m (€34.77m) to 6,400 borrowers after the UK's Court of Appeal last week found against the building society.
The court said the building society was not allowed to break the terms that said the mortgages would track the Bank of England's base rate.
The buy-to-let investors had been reaping the benefit of record low interest rates.
But West Brom added 1.99pc to landlords' tracker rates, despite the base rate staying at 0.5pc.
Mr Alexander said: "There are different terms in the Bank of Ireland contracts, but the principle is the same.
"Last week's decision cost West Brom £27.5m. We are looking at a £50m [€63m] claim in the case of Bank of Ireland."
The head of the Propety118 Action Group said he is seeking £600 each from 1,000 affected Bank of Ireland investors to complete the funding for the action against the bank.
Bank of Ireland said: "Bank of Ireland's offer document and mortgage terms and conditions expressly stipulated that the tracking margin or differential could be varied, and the offer and mortgage conditions documents are consistent, allowing for the differential to be lawfully changed."