Its not a sharingan.
I dont know what her eyes are. And I dont know if its a Doujutsu Kekkei Genkai or not but her red eyes do look like tomoe-less sharingans. The databook says that her skill in genjutsu rivals Itachi's. And thats saying a lot. The second Mizukage pointed out that possessors of the Yin-release tend to be good at genjutsu. And, all Uchiha are Possessors of Yin-release, hence their skill with genjutsu. It wouldn't be a surprise to me if Kurenai, who's quite a pro at genjutsu, too was a possessor of the Yin release. Non-Uchiha's are also good at Yin-release, the Nara clan and the Second Mizukage being examples. So this doesn't necessarily mean that her eyes are related to the Sharingan somehow.
But this does mean that she has two things in common with the Uchiha. Red eyes and a mastery of Yin release.