Fionnan Sheahan: What did she hope to achieve by this?
In his absence, Mr Gilmore has lost a junior minister -- the second such resignation since entering office following the 2011 General Election.
Except Roisin Shortall's departure is different from that of Willie Penrose 10 months ago as it was directly caused by a clash with their coalition partners.
The simmering tensions between Ms Shortall and Dr James Reilly finally boiled over in the past week with the inevitable conclusion of her departure from office.
The personality and ideological differences escalated to such an extent that the working relationship became impossible to reconcile.
The Dublin North-West TD is now out in the cold.
What exactly is achieved by her resignation isn't actually clear at all.
She felt frustrated at not being able to wield influence inside of Government.
Outside the door, she'll have no influence at all.
When Labour entered Government 18 months ago, she had ambitions to be a cabinet minister.
Clearly Mr Gilmore felt he needed the experience of senior figures who had sat at the cabinet table before, given the difficult decisions needing to be taken by the Government
Her subsequent handling of the task she was handed would appear to vindicate this decision.
She missed out on promotion to the cabinet table for the second time this year upon Mr Penrose's departure from the Super Junior perch.
The dissatisfaction of not getting to the top table festered.
Still, she was appointed to a reasonable junior ministry in the Department of Health, in charge of primary care -- technically, at least.
After all, she was the only junior minister to have a special adviser appointed to assist in her role, which was regarded as a sign of Labour's commitment to the task.
Her area of responsibility though is core to the overall healthcare agenda so she was always going to have to work closely with Dr Reilly.
The pair proved incapable of working together. Blame Dr Reilly's boorish demeanour perhaps, but he's the senior minister, so he was always going to come out on top.
It's never a good sign when people working in the same building are constantly writing letters to each other.
The in-tray and out-tray on Dr Reilly's desk appears to be constantly full. The deterioration of their interaction is catalogued in a series of letters between their offices.
The first signs emerged late last year with a stream of emails over the development of the free GP care, where Ms Shortall wasn't happy with the progress of the policy.
The spats continued over the following months with the animosity palpable.
Last week, Ms Shortall came to a crunch decision as Dr Reilly faced a motion of no confidence.
In her keenly anticipated contribution, Ms Shortall failed to mention the health minister by name. Instead, she delivered a critique of the policy failings of the department. Her colleagues in Government were none too impressed by her petulant speech.
Ultimately though, she did vote confidence in Dr Reilly.
The tensions mounted further later in the week when it was revealed he added two towns in his own Dublin North constituency to a list she had developed of potential sites for health care centres. Ms Shortall made it clear she didn't approve of his additions and didn't know or understand his rationale.
Labour Party ministers weren't rushing to her defence and stood by Dr Reilly.
The emergence of the documentation, via a remarkably precise Freedom of Information request, also raised eyebrows on both side of the coalition.
A meeting this week to clear the air didn't work that way and was described as "tense and difficult".
The clash over the primary care units turned into a personal crusade that missed the wider picture.
Dr Reilly had the last laugh when a further letter emerged yesterday contradicting her claim she didn't know why he had changed the rules.
The relationship plunged to new depths.
Teeing it up as a point of principle -- on an issue her colleagues had moved on from -- meant matters were coming to a head.
Her resignation last night lanced the boil and there were few tears being shed within the coalition.
The embarrassing public squabble undermined confidence in the Government's ability to deliver on the much-needed reforms of the health service.
Dr Reilly wins the battle -- although not yet the war.
And Ms Shortall merely joins the ranks of the disaffected Labour TDs.