Karen Bradley, who represents the English constituency of Staffordshire, was formerly the Secretary of State for Culture, Media and Sport.
"In the last few days I have been told that I have a small lesion in my right lung which needs to be removed", Brokenshire wrote in his resignation letter. I recognise, however, that as a result of my forthcoming surgery I will not be able to give the effort, energy and complete focus needed at this important time if we are to secure the positive outcomes both you and I are committed to achieving.
Northern Ireland has been without a regional government for nearly a year, destabilising the delicate balance between Irish nationalists and pro-British unionists that has been shaken by Britain's June 2016 vote to leave the European Union.
But in arriving at a point when everything appears to be broken it will be hard for Mrs Bradley to be accused of making things much worse.
Sources say that he is citing "ill-health" as the reason for his resignation.
Mr Coveney is now on a visit to the Middle East and will stop over in London on his way back to Ireland.
Amid the chaos and confusion that attended this week's reshuffle, the announcement of a new secretary of state for Northern Ireland passed off with little notice.More news: Photos show devastating aftermath of auto park fire
He said: "His unwavering commitment - in public and in private - over the a year ago to securing the effective operation of the devolved powersharing institutions in Belfast has been hugely important".
Theresa May will now have to replace the 50-year old MP, who has been doing the Northern Ireland job since July 2016.
The news was confirmed by the Northern Ireland Office this morning (Monday).
DUP leader Arlene Foster congratulated the MP and said she welcomed her to the "friendliest part of the UK".
Mr Brokenshire bore no responsibility for the breakdown between Stormont's two parties, but his response to the situation drew criticism.
"I can assure you nobody is more disappointed or indeed frustrated with the fact that we haven't as of yet struck a deal - that we don't have the institutions back live - than all of us in Sinn Féin", she said. I send my best wishes to him and the entire Brokenshire family.
"James leaves the role with a very intimate knowledge of Northern Ireland and I look forward to working with him again in the future".