You only have to tell the mortgage provider if your loan documentation says you must.
Telling the insurance and registering with the PRTB are extremely important as the previous poster stated plus the new NPPR. You also need to learn about how to be a landlord and how to work on your rental income for tax purposes. Look up AAM investment section and revenue.ie to give you an idea of what to do.
Depending on when you purchased there may be a stamp duty requirement.