I have a large dark brown birthmark with hair on it should i be concerned?

No: Birthmarks are very common and rarely represent something sinister. They can be a cosmetic nuisance depending on their location. The only time you should be concerned is if you notice any new changes like increasing size or color, irregular borders, any bleeding or discharge etc in the area. Otherwise just let it be.

Maybe,maybe not : Most people have their own idea of what "large" means so your description is not very specific. Most brown birthmarks will have a little hair and be benign.One called a giant hairy nevus or bathing trunk nevus has some potential for areas that transform to a malignancy.If you haven't shown it to your doc, you should.