The Financial Services and Pensions watchdog upheld 127 complaints by customers in 2018.
The Financial Services and Pensions Ombudsman (FSPO) has published a list of cases where compensation was awarded.
It is the FSPO's job to try and resolve a complaint or dispute when mediation efforts have not worked with a financial service or pension provider.
Last year, 234 legally binding decisions were made and 127 of them were either fully, substantially or partially upheld.
They have the power to instruct a provider to award compensation of up to €500,000.
One case saw a couple get compensation of €90,000 after their mortgage provider adopted what the ombudsman called "an obstructive approach" after they fell into arrears with a buy-to-let mortgage.
In another case, €3,750 in compensation was awarded to a customer whose bank opened a new account without their consent or knowledge.
Another case saw a driver awarded €3,000 after he lost his no-claims bonus having reported a crash caused by an uninsured driver.
The Ombudsman said it is clear from the decisions made today that some providers do not always act in the best interests of their customers.
Ger Deering from the watchdog said: “I am very pleased to publish the legally binding decisions made during 2018. I welcome the fact that the majority of complaints were resolved through the informal mediation process we provide as this delivers a faster outcome that is acceptable to both parties.
"In 2018, we resolved approximately 2,300 complaints through this informal mediation process.
"However, it is clear from the decisions I am publishing today that some providers do not always act in the best interest of their customers. I have found that it is still the case that some providers are not willing to listen to or engage sufficiently with their customers in order to resolve disputes.
"Where disputes are not resolved by agreement, I will continue to use the extensive powers available to me to investigate and adjudicate complaints in a transparent and impartial manner to ensure fairness and, where necessary, to direct providers to pay compensation or rectify their conduct.”