When forming (or creating) a PLC there must be:
at least £50,000-worth of share capital of which at least 25% must have been paid for.
two shareholders
two directors, one of whom may also be the company secretary
a certificate of entitlement (the trading certificate) to do business and borrow capital
While it is not compulsory for a PLC to "float" its shares (some PLCs retain ownership of all their shares, maintaining the PLC designation for the extra financial status), many do so, and their shares are usually traded on either the London Stock Exchange or the Alternative Investments Market (AIM). Irish public limited companies usually trade on the Irish Stock Exchange, though many also list on the LSE, or more rarely, the AIM.