Firstly start by doing research on any legislation for STRs in Northern Ireland
then do some market research to identify set up and ongoing costs for having an STR including utilities, insurance, cleaning etc.
Look at what competitor hosts are charging for similar places. Estimate how many days a month you are likely to let your place for.
put cost and likely income into a profit and loss budget. This will give you an idea of whether it will be more profitable to do STR rather than LTR. @Paul9248z