D. Tiernan was a Starfleet officer in the 23rd century.
Tiernan was a member of the Science Ops team listed on the dedication plaques of the USS Shenzhou and the USS Discovery. (DIS: "The Vulcan Hello", dedication plaque; DIS: "Context Is for Kings", dedication plaque)
This character was only mentioned in writing.
This officer was named after director of photography Darren Tiernan.