His going will raise few eyebrows in political circles following the party threat to take disciplinary action against him over his evidence to the Flood Tribunal.
While there has been no finding of wrong-doing against Mr Cosgrave, he is yet another politician to fall victim to the probe into financial links between elected representatives and developers involved in controversial land rezoning decisions in North County Dublin.
By falling on his sword, he ends a family dynasty that included two former taoisigh. He is the son of former taoiseach Liam Cosgrave and grandson of the late taoiseach William T Cosgrave.
His resignation was probably inevitable after Fine Gael indicated last month that it intended taking disciplinary proceedings against Mr Cosgrave, possibly including the ultimate sanction of dismissal, after it emerged he had misled an internal party inquiry into political donations.
The Flood Tribunal found Mr Cosgrave had received over £7,000 in election payments and other payments from former lobbyist Frank Dunlop. He had declared a sum of £3,000 to £3,500 to the Fine Gael inquiry.
A TD or senator for 20 years, his bid for a Dáil seat in Dun Laoghaire-Rathdown failed last year.
When Fine Gael added a third candidate to the ticket he accused the party of trying to erase the family name.
With the tribunal continuing to bring fresh evidence to light, it remains to be seen if other politicians will take a similar course of action.
So far Fianna Fáil has given no indication that it is prepared to take disciplinary action against any members of the party who are exposed by the glare of the Flood spotlight.