Yes, in most cases it is possible to tattoo over scar tissue.
More Info: It is important to seek out an experienced tattoo artist familiar with tattooing over scars. Certain types of scarring such as keloids should not be tattooed because the tattoo needle can actually aggravate the keloid, triggering further inflammation. If you are prone to keloids, you should avoid tattooing completely since there is a chance of more keloids forming on the tattooed area. Body piercing is also not recommended as there are cases where keloid prone people have suffered golf-ball sized keloids simply from having their ears pierced. Some severe burn scars may not be suitable for tattooing either because they have left the skin too thin and fragile to accept the tattoo needle and hold the ink.
Generally, the longer a scar has been present, the better the result will be. Though there is no formal medical research on the subject of tattooing over scars, the general consensus in the tattooing industry is that a scar should be at least a year old before it is tattooed over, and even longer for especially deep or slow-healing scars. Each case is unique, so again, it is important to find an experienced tattoo artist who will know how to properly evaluate the scar tissue and make informed suggestions on the most effective types of designs and colors to utilize in order to best camouflage the scar and somehow even incorporate it into the design.