The Government might not take delivery of its new jet until shortly before Ireland's EU presidency begins in January.
While tenders said the jet should be in service by October, Government sources said there was a growing perception that that deadline might not be met.
Government sources attributed a delay to an eight-week evaluation of tenders by the Department of Defence, which is not expected to produce its initial findings for another two to three weeks.
The evaluation process is made more complex by the fact that the Government has said it is open to the purchase of second-hand aircraft.
However, the indication from well-placed Government sources that Ministers might have to wait until the end of the year for the jet might be interpreted as sign that they favour the purchase of a new aircraft at this stage in the process.
When asked about evaluation, the Department of Defence said it was still feasible to meet the October deadline.
The department's spokesman said: "There's no guarantee that it is do-able. There could well be companies with partially-built aircraft. I certainly wouldn't rule out October to get it up an running."
The Government said in separate advertisements in the EU Journal that it wanted to acquire one 45-seater aircraft in place of the 14-seater Gulfstream, and a smaller seven-seater aircraft to replace the five-seater Beechcraft. If purchased new the two aircraft could cost up to €70 million.
It is believed that Airbus and Boeing, the two main plane-makers, are both in the running for the contract for the bigger jet. The identity of other bidders is not known.
Amid tighter public finances leading to the closure of hospital beds, the decision to purchase the aircraft was criticised by the Opposition. But the Government, which had publicised a series of breakdowns of the Gulfstream jet, has said it needs a bigger aircraft to enable it to fulfil it duties under the EU presidency.
The tender for the larger aircraft said the layout must include a separate forward compartment with eight seats. The tender for the smaller aircraft says it should include an air ambulance option capable of taking a stretcher unit with oxygen. Entertainment and refreshment facilities were also required.