Unless you are in a profession where the Revenue allows an earlier retirement age than 60, you can only retire at 50 if your pension is through an occupational pension scheme and the rules of that scheme permit retirement at age 50.
If it's a defined benefit scheme, there's a strong likelihood that you won't be able to take early retirement because of solvency issues. Even if it's defined contribution, it is likely that you will only be able to take part of your pension as a tax free lump sum (depends on how much is in the fund, your salary and how much service you had with the employer).
Having said all that, if you are able to draw your lump sum at 50, there's nothing to stop you using that money to set up a business and keep working.