Who has achieved enlightenment?
Keeping this in view, how is enlightenment achieved?
In Buddhism, enlightenment (called bodhi in Indian Buddhism, or satori in Zen Buddhism) is when a Buddhist finds the truth about life and stops being reborn because they have reached Nirvana. Once you get to Nirvana you are not born again into samsara (which is suffering).
Furthermore, has the Dalai Lama achieved enlightenment?
Dalai Lamas are believed to be the reincarnation of Avalokitesvara, an important Buddhist deity and the personification of compassion. Dalai Lamas are also enlightened beings who have postponed their own afterlife and chosen to take rebirth to benefit humanity.
When you achieve nirvana, you stop accumulating bad karma because you've transcended it. Once you have fully escaped the karmic cycle, you achieve parinirvana -- final nirvana -- in the afterlife. As with Hindu nirvana, souls that have achieved parinirvana are free of the cycle of reincarnation.