The changes will see fixed rates rise by between 0.25 percentage points and 0.45 points, depending on loan-to-value bands.
The revised rates will apply to three-year and five-year fixed rate products.
ICS, which is the trading name of Dilosk, said the changes reflect prevailing market conditions.
Dilosk bought the ICS brand from Bank of Ireland.
Existing fixed-rate customers will not be affected, the lender said.
It is the first lender to hike rates this year, although it is a niche player in a mortgage market dominated by AIB and Bank of Ireland.
The move was not expected as the European Central Bank (ECB) is expected to keep its lending rates on hold.
ICS Mortgages chief commercial officer Ray McMahon said: “While we continually seek opportunities to support borrowers through competitive pricing and innovative products, these fixed rate adjustments reflect the realities of the increase in swap market pricing over the past two months.”
Dilosk gets its funding from the money markets.
This is unlike the banks that have vast reserves of customer deposits and also tap the ECB for funding.
Mr McMahon added: “We remain committed to supporting our customers and broker partners through clear and timely communication.”
He said ICS Mortgages continues to offer a broad range of mortgage solutions through both our direct channel and its nationwide broker network.
“Our focus remains on delivering flexible, innovative products and a high standard of service to meet the evolving needs of today’s homeowners,” Mr McMahon said.
Michael Dowling, of Irish Mortgage Brokers, said the announcement a of rate increases up to 0.45 percentage points by ICS is not welcome news for customers of ICS.
He said the move will mean a €300,000 mortgage will be up to €80 more expensive following the increase.
But he does not expect other lenders to hike rates.
“I do not believe other banks will follow suit as the reason for the increase relates to how ICS Mortgages sources money to fund its loan book.
“These increases highlight the risk associated with a lender who funds its loan book from the market rather than the mainstream lenders who source their funding from the significant deposits they have.”
Mr Dowling said in recent weeks Avant Money, through its Spanish owner BankInter, and MoCo, through its Austrain owner Bawag, have started offering deposit accounts to customers to help fund their mortgage book.
The European Central Bank (ECB) held steady at its December meeting, having cut rates eight times since June 2024.
This week mortgage broker Doddl.ie said it expects more competition is expected in the home-loans market this year, but mortgage rates are expected to remain stable.
Lenders are expected to make selective adjustments to mortgage rates rather than engaging in major price cuts, according to Martina Hennessy, of digital mortgage broker Doddl.ie.
Meanwhile, estate agency DNG predicts further low to mid-single digit house price increases this year.
It said second-hand house prices in Dublin rose by 5.2pc last year. The average price of a resale Dublin property now stands at €613,078, DNG said.
It said there is what it calls Significant increase in the number of investors leaving the rental sector in H2 2025.