RTÉ recorded a net deficit of €2.8m last year, which was influenced by worse-than-expected growth in licence fee revenue and the adverse impact of foreign currency.
This compares to a surplus of €200,000 in 2014, while in 2009 the national broadcaster had a deficit of €16.5m.
RTÉ's operating profit in 2015 of €14m represented a fall of over €2m on the previous year.
According to the organisation's annual report, commercial revenue grew by 4% to €155.4m in 2015, buoyed by an overall improvement in economic conditions as well as strong performances in the areas of TV, radio, and digital advertising & sponsorship.
Annual licence fee revenue received by RTÉ between 2014 and 2015 rose by €300,000 to €178.9m, however, the broadcaster said this figure was a "disappointing outcome" given "the general overall improvement in economic conditions and the growth in net new household formations".
The report also highlighted the increase in the number of homes without a TV set as a "growing concern".
Overall, licence fee income received by RTÉ through public funding has declined by around 11% (€22m) since 2008.
On costs, the report said: "Following significant reductions in operating costs between 2008 and 2013 of over 30%, inevitable cost pressures arise, particularly in personnel related operating costs.
Operating costs of €320.3m increased by €8.4m between 2014 and 2015, however, operating costs for the year were still 27% lower than 2008 levels.
Meanwhile, the negative impact of both strong dollar and pound rates meant the adverse impact of foreign currency on RTÉ's 2015 results was €2.6m.
Commenting on the report, Deputy Director-General RTÉ and Managing Director of RTÉ News & Current Affairs Kevin Bakhurst said: "Over the past five years we have made huge progress in reshaping RTÉ for the future.
"Costs remained substantially down on 2008 levels and successive independent reviews find RTÉ's operations to be efficient.
"Yet, while we managed to deliver some increases in commercial revenue, overall the public funding base remained static and is, in real terms, declining," he added.
Minister for Communications, Energy and Natural Resources Denis Naughten said RTÉ's performance "should be viewed in the context of ongoing reductions in public funding in recent years and the competitive commercial market in which it operates.
The minister highlighted that investment in its digital services will be critical if RTÉ is to maintain its relevance and compete with larger multinational media organisations in the future.
Mr Naughten acknowledged that high levels of TV licence evasion - which stood at 14% last year - and the challenges facing the existing TV licence system needed to be addressed as a matter of urgency.
