The Government was offered a special deal to buy two new aircraft for more than €8m but opted to buy only one amid concerns including a shortage of available hangar space.
The new Pilatus PC-12 NG was purchased in March under special EU rules that allow normal procurement practices to be put aside in "crisis situations".
Among the reasons put forward for its purchase were maintenance concerns around the government Learjet and the fact that two other Air Corps planes had amassed more flying hours than almost any comparable aircraft in the world. Internal Department of Defence records reveal concerns were raised about how the Taoiseach and other senior ministers would be able to leave the country at short notice given the lack of available State aircraft.
A "security sensitive" briefing note - which has been released under FOI - said emergency travel might be required arising from the Covid-19 pandemic or other "Brexit-related travel".
The note said: "Commercial travel in the current developing situations may be unavailable, unreliable, insecure or too risky to contemplate."
The Department of Defence had been offered two of the PC-12 aircraft by Pilatus for around €4.3m each, before VAT, and were told they were ready for immediate delivery.
A business case was put forward for acquiring both of the planes, either by purchase or through a lease arrangement. The document outlined how EU laws allowed for the "award of certain contracts at speed in an emergency situation" such as the Covid-19 pandemic.
Internal emails show the Secretary General of the Department of Defence was "comfortable" with the idea of acquiring one new aircraft but "not necessarily convinced" of the rationale for two.
A later business case said one aircraft - purchased for €5.23m including VAT - would actually be better because of restricted hangar space available at Casement Aerodrome in Dublin.
It also said purchase of both planes "would impact on pilot availability, particularly in light of pilot training requirements".
A spokeswoman for the Department of Defence said the new Pilatus airplane had been bought to "provide very immediate additional fixed wing capacity, and enhanced contingent capability, to meet the unique situation arising from the Covid-19 pandemic".