The building follows an eclectic architectural style, while the museum includes paintings of national and international significance, furniture, porcelain, jewellery and a neo-Gothic organ.

O'Neill was a Portuguese aristocrat of distant Irish descent who had multiple business interests including Portugal's match monopoly.

The building employs several architectural styles, adopting a Revivalist approach that includes Neo-romanticism, Neo-Gothic, Neo-Manueline and Neo-Moorish.

Notable were the purchases of a neo-Gothic organ, built for the Count, and rare 16th Century manuscripts including the valuable "Chronicle of Don Afonso Henriques", the first king of Portugal, written by Duarte Galvão (1446-1517).

Directors have included João Couto, Carlos Bonvalot, Branquinho da Fonseca and Maria Alice Beaumont.