Loan to Income is the ratio between the amount of loan advanced and the customer(s) income. e.g. the current Central Bank guidelines allow for customers to borrow up to 3.5 times the gross annual salaries i.e. if gross income is €50,000 then they can borrow €175,000. There are exceptions to these rules and you can discuss this with your mortgage advisor.
Central Bank rules on loan to income do not apply for a straight mortgage switch where you are not increasing the balance outstanding on the mortgage.