Heat the Guinness with the lemon juice, but do not bring to the boil. Mix the grated cheese (e.g. Gabriel, Desmond or Ardrahan) and the cornstarch and add gradually while stirring constantly. Season to taste and simmer (do not boil) until the fondue thickens.
Place on a fondue plate. Skewer the vegetables on fondue forks and dip into the fondue.
A rustic farmhouse bread or pain de campagne can be served with it.