The alternative is having an elected president to act as head of state - other heads of state have varying degrees of power, if any, so presumably if the country became a republic there would be a significant debate on what an elected president's powers would be, length of term of office etc.
Also, salary. The Irish president currently earns just over 249 thousand euro. It was reduced recently under a public pay cap piece of legislation - all public sector salaries are capped at 250 thousand euro.
The role has very limited constitutional functions, and is largely ceremonial, though the unimpeachably admirable Mary Robinson, the president before last, raised its profile hugely, bringing to it all her previous work in the commission for gay law reform, campaigning for the liberalisation of contraception laws, the lifting of the marriage bar for female civil servants, met with Gerry Adams, Hume, Trimble and the Queen in an effort to put Anglo-Irish relations on an easier footing etc. Became UN Commissioner for Human Rights after she left office.
To stand for president, you have to be an Irish citizen of over 35, and to be nominated by 20 members of the Oireachtas or 4 county councils.