Dividends are declared in cents, not in percentages.
So let's say Bank of Ireland pay 50 cents per share once a year.
If the share price, today, is €10, then the dividend yield is 5% ( 50/1000)
If the share price of Bank of Ireland rises to €20, then the dividend yield will fall to 2.5%.
But the key to understanding this is that the company pays a dividend per share in cents. They don't pay percentages. That is calculated by you or by analysts afterwards.
Additional points
The 50 cents is gross. They will deduct 10 cents Dividend Withholding Tax and you will receive 40 cents.
Most companies pay dividends twice a year. I have used once a year in the example for simplicity.