The ad, which has the headline "low fares finally in Belfast", shows a picture of Sinn Fein's Martin McGuinness and Gerry Adams being interviewed by the press.
McGuinness is pictured with a speech bubble coming out of his mouth reading "Ryanair are so low even the British Army flew home". It ran in the Belfast Telegraph.
Four people complained, including Michael Copeland, a member of the Northern Ireland Assembly.
The complainants variously said that the ad trivialised and mocked the role of the British Army in Northern Ireland; that it would distress the families of those killed; and that it trivialised the province's violent history.
Copeland, a member of the Ulster Unionist Party, also challenged the wording of the ad, saying that it implied that the British Army was not "at home" in Northern Ireland.
The Advertising Standards Authority said that although it understood that complainants found the ad "tasteless in the context of the recent political history of Northern Ireland", it considered that it would be unlikely to cause serious or widespread offence.
In its ruling, it said: "We considered that, in the context of an ad for flights departing from Belfast, the use of the picture of two well-known Sinn Fein politicians and the reference to the British Army would be understood by most readers as a light-hearted reference to the recent withdrawal of troops from Northern Ireland."
Ryanair's "home-made" ads often cause offence among members of the public and result in discussions with the ASA.
This year it has been rapped for a campaign describing then-chancellor Gordon Brown as "the great plane robber" and "greedy Gordon wins Oscar... for best stick up".
In 2005, it managed to make the list of the most-complained about ads of the year, with one headlined "London fights back", alongside a picture of Winston Churchill, which appeared in the press only eight days after the July 7 bombings.