Oloroso is probably the main type used. All Midleton sherry casks are Oloroso
Here is a break down from http://en.wikipedia.org/wiki/Sherry
('fine' in Spanish) is the driest and palest of the traditional varieties of sherry.
is a variety of fino sherry made around the port of Sanlúcar de Barrameda.
is a variety of sherry that has been aged first under a cap of flor yeast, and then is exposed to oxygen, which produces a result darker than fino but lighter than oloroso.
('scented' in Spanish) is a variety of sherry aged oxidatively for a longer time than a fino or amontillado, producing a darker and richer wine.
is a rare variety of sherry that is initially aged under flor like an amontillado, but develops a character similar to oloroso, with some of the richness of oloroso and some of the crispness of amontillado.
(Jerez Dulce in Spanish) is created when one of the preceding varieties of dry sherry is sweetened with Pedro Ximénez or Moscatel wine.
is a common variety of sweet sherry made from oloroso, with other varieties including pale cream sherry
(made from fino) and medium sherry
(made from amontillado).