Leo Varadkar has been in written communication with Melania Trump - but he is refusing to release details of the letter because it involved "international relations".
The note to the American First Lady is just part of correspondence between the Taoiseach and the White House that will remain confidential.
On foot of a Freedom of Information (FoI) request, Mr Varadkar's department confirmed it had written to President Donald Trump and Vice-President Mike Pence five times last year.
Mr Varadkar was in touch with the First Lady in April last year.
Earlier this year, he had an exchange of pleasantries with Mr Pence's mother, Nancy, whose father emigrated to the US from Co Sligo.
Only the letter to Mrs Pence has been released.
Officials cited an exemption under FoI laws that allows them to refuse communications involving international relations.
Mr Varadkar and Nancy Pence met during the Taoiseach's visit to Washington for the St Patrick's Day festivities in 2018.
In her letter to Mr Varadkar earlier this year, Mrs Pence wrote: "Dear Taoiseach, it has been some time since we shared a lovely time at the home of the Vice-President.
"It was lovely to meet you. Thank you for the beautiful scarf and Irish pin."
Mr Varadkar responded on St Patrick's Day this year saying that he "greatly enjoyed" meeting her at the breakfast hosted by Mr Pence and his wife, Karen.
"Your pride in the Vice-President was obvious and he spoke with great passion about his Irish roots," he added.
"I hope this finds you well and I want to wish you every happiness. Happy St Patrick's Day."
Mr Pence hosted Mr Varadkar and his partner, Dr Matt Barrett, for the traditional St Patrick's breakfast at his official residence, the US Naval Observatory in Washington DC last month.
Mr Varadkar renewed an invitation for Mr Pence to visit Ireland.
He also said: "You absolutely must bring your mother… I had the pleasure of making her acquaintance last year. We've been in correspondence once or twice since then.
"One thing I found really extraordinary and really memorable is that she has some words of Irish."
Separately, Mr Varadkar was in contact with Mr Trump in March, May and November last year. The correspondence with Mr Pence was in March and June.