Taxpayers paid more than €2.9m for pre-paid envelopes for TDs and Senators in the past three years.
Last year, Oireachtas members used more than 1.1 million of these envelopes.
The new figures have been released under the Freedom of Information (FOI) Act.
A TD is entitled to 625 letter size pre-paid envelopes a month, while a Senator is entitled to 375.
They can be used for responding to queries from constituents, but not for campaigning or party purposes.
This cost more than €947,000 last year and just under €1m in both 2017 and 2018.
Last year, 63 TDs claimed their full allowance - including Taoiseach Leo Varadkar, Tánaiste Simon Coveney and 11 other Cabinet ministers.
The only senior ministers who did not use their full batch of envelopes last year were Shane Ross and Katherine Zappone.
Dara Murphy quit as a Fine Gael TD in December, and for the two years before he was controversially based mainly in Brussels.
The new figures show he claimed his full allowance of Oireachtas envelopes between January and November of last year.
Some 24 Senators claimed their full allowance last year.