Why do members of the Irish Parliament have two letters TD after their names?

"A TD (full Irish form Teachta Dála) is a member of Dáil Éireann, the lower house of the Oireachtas (the Irish Parliament). It is the equivalent of terms such as "Member of Parliament" (MP) or "Member of Congress" used in other states. The official translation of the term is "Deputy to the Dáil", though a more literal translation is "Assembly Delegate"."

https://en.wikipedia.org/wiki/Teachta_D%C3%A1la

So in simple terms, it's the equivalent of "MP" or similar terms for Parliamentarians.

For electoral purposes, the country is divided into areas known as constituencies, each of which elects either three, four or five TDs. Under the Constitution there must be at least one TD for every 20,000 to 30,000 people.[4] A candidate to become a TD must be an Irish citizen and over 21.[5] Members of the judiciary, the Garda Síochána and the Defence Forces are disqualified from membership of the Dáil.[5]

There are 166 TDs in the 31st Dáil. The next general election will elect 158 TDs, a reduction of 8, following the passing of the Electoral (Amendment) (Dáil Constituencies) Act 2013.[6]

Source of Reference:- Wikipedia - Teachta Dala