An Post has secured approval to hang two large new banners outside the GPO on Dublin's O'Connell Street.
It is part of plans to reverse falling visitor numbers to its flagship 1916 exhibition, despite opposition from a leading heritage group.
An Bord Pleanala has rejected an appeal by An Taisce against the decision by Dublin City Council to grant planning permission to hang the 6.5-metre banners from the portico to promote its Witness History Visitor Centre.
The board also extended the duration of the permission from two to three years, as well as sanctioning a replacement of an existing brass plaque on the side of the building.
An Post believes the new signage is necessary to raise awareness of the €10m attraction, which was formally opened in March 2016 as part of the centenary commemorations of the Easter Rising.
Subject to a number of planning conditions, the board said the site-specific nature and purpose of the banners to advertise the exhibition, which highlights the history of the GPO itself, would not detract significantly from the character of the building.
Nor would it take from the O'Connell Street Architectural Conservation Area, it said.
It also noted that it was a policy of the Dublin City Development Plan 2016-2022 to promote tourism within the city.
An Taisce had claimed the banners conflicted with council policy regarding protected structures, protection of views and the integrity of city landmarks.
It claimed the size and prominence of the banners would have "an obtrusive effect" on the GPO and O'Connell Street.
An Taisce said the use of the banners was "unnecessary and undesirable".
However, An Post claimed the Witness History exhibition has been suffering falling numbers since it opened in 2016, and it regarded the absence of good signage as "a substantial drawback".
The facility attracted 160,833 in 2016, but numbers have declined from 100,343 in 2017 to 87,899 last year.
An Post said the impact of the banners on the GPO building would be temporary and minor.
The company noted that banners had previously been permitted to advertise historic events at the GPO, as well as at other protected structures such as the Bank of Ireland on College Green.