I've tried putting this concealer on with my ring finger but it still doesn't seem to cover my very dark under eye circles. So I was wondering could I use a concealer brush to put this type of concealer on? Would that cover them up better? Thanks in advance..

Its better to use your fingers as the warmth of your fingertips slightly melts the concealer helping it to blend better, however you may be pressing to hard or are not perhaps skilled with your hands - resulting in the lack of coverage you describe. Id try blending with more concealer on your fingers and more lightly however if this fails- go for the brush .

Whenever i use a brush to apply concealer i get it out of the pot with my finger and put it onto the back of my hand (this warms it- helping towards a better coverage as described before) than brush it from there.

Hope this helps.