1) Check with the company to see if they offer a share dealing service.
If they don't...
2) You will need to ask one of the bricks and mortar stockbrokers like Davy's or Goodbody to sell them for you
3) It might be possible to switch the shares from share certificates to online shares. But you need to ask the company that.
If this was a company whose Irish subsidiary employed you, then ask the Irish company or some employees of the company how they managed to sell the shares.