But Mr White, who replaced Ms Shortall as Labour's junior health minister, is said to have a far better working relationship with Dr Reilly.
The uncertainty over her functions was one of the reasons behind Ms Shortall's resignation in the wake of the primary care centre controversy.
Like her, Mr White is in charge of primary care, the extension of GP care and GP training – but not legally.
Junior ministers have delegated powers specifically saying what they are in charge of.
In addition to those legally prescribed functions, Dr Reilly has also signed a written agreement saying Mr White is responsible for the other functions, described as "areas that don't lend themselves to statutory delegation".
It means the senior minister still officially has the decision-making power over those areas.
Ms Shortall got a similar letter last year, saying she was in charge of the primary care and GP areas.
Yet Dr Reilly was able to overrule her on the selection of the primary care centre sites, when he added two locations in his own constituency.
A government spokesperson denied the need for the written agreement was a sign of a lack of trust between Dr Reilly and Mr White.
"This is Minister White working with Minister Reilly," the spokesperson said.
Copies of the two letters were not available last night for comparison.
Meanwhile, the number of drug task forces is to be reduced from 24 to 19 as part of a new shake up.
Mr White said the cuts follow a review of the task forces which were set up in the late 1990s to help in areas hit by abuse, particularly heroin addiction.
The review will see the task forces renamed as Drug and Alcohol Task Forces and they are to get clearer terms of reference as well as more explicit guidelines on how they should be run. There is also to be better monitoring of how the €28m to be allocated to them next year will be spent.