For electoral purposes, the country is divided into areas known as constituencies, each of which elects three, four, or five TDs. Under the Constitution, every 20,000 to 30,000 people must be represented by at least one TD. A candidate to become a TD must be an Irish citizen and over 21 years of age. Members of the judiciary, the Garda Síochána, and the Defence Forces are disqualified from membership of the Dáil.
The term was first used to describe those Irish parliamentarians who were elected at the 1918 general election, and who, rather than attending the British House of Commons in London, to which they had been elected, assembled instead in the Mansion House, Dublin on 21 January 1919 to create a new Irish parliament: the First Dáil Éireann. Initially the term "Feisire Dáil Eireann" (F.D.E.) was mooted, but "Teachta" was used from the first meeting. The term continued to be used after this First Dáil and was used to refer to later members of the Irish Republic's single-chamber Dáil Éireann (or "Assembly of Ireland") (1919–22), members of the Free State Dáil (1922–37), and of the modern Dáil Éireann.
The initials "TD" are placed after the surname of the elected TD. For example, the current Taoiseach (head of government) is "Leo Varadkar, TD". The style used to refer to individual TDs during debates in Dáil Éireann is the member's surname preceded by Deputy (Irish: an Teachta): for example, "Deputy Martin", "an Teachta Ní(Bhean Úi) Mháirtín" or "an Teachta Ó Máirtín"
After controversy regarding alleged abuses of the Oireachtas expenses provisions, the system was simplified in 2009 and 2010 into two allowances:
Travel and Accommodation Allowance – ranging from €9,000 for TDs less than 25 km from Leinster House to €34,065 for those more than 360 km away.
Public Representation Allowance – for maintaining a constituency office; €20,350 for backbench TDs, less for ministers. All expenses must be vouched, except for a "petty cash" allowance of €100 per month. Until December 2012 TDs could choose between a €25,000 vouched allowance or €15,000 unvouched.
^See e.g."Take Charge of Change"(PDF) (in English and Irish). Dublin: Office of the President. 2012. pp. 3, 7. Archived from the original(PDF) on 28 March 2013. Retrieved 14 August 2013. Empower local Government so that our national representatives TDs don't engage in parochial politics./Cumhacht a thabhairt don Rialtas áitiúil ionas nach mbeidh ár gcuid ionadaithe náisiúnta, na TDanna, i mbun na polaitíochta paróistí.
Phillips, Walter Alison (1922). "Ireland". Encyclopædia Britannica. 31 (12th ed.). p. 573. Retrieved 21 October 2016. The victorious group assumed the title of the Irish Republican party, and styled themselves not M.P., but F.D.E. (Feisire Dail Eireann, i.e. members of the Assembly of Ireland).;
^"Prelude". First Dáil proceedings (in Irish). Oireachtas. 21 January 1919. c.9. Retrieved 21 October 2016. Tháinig na Teachtaí I gceann a chéile I nÁrus Árd-Mhéire Bhaile Átha Cliath ar a 3.30 iar nóin.