For our contemporaries, the Pyramids of Giza is a completely unique remnant of the Ancient World, otherwise irretrievably lost. Obviously, the Antiquity must have promoted the Pyramids to the status of “wonder” for altogether different reasons. Judging by the list of the Original Seven, ancient people would set store by size, functionality or, ideally, a combination of the two. In some cases, the practical (the Lighthouse of Alexandria) or spiritual (the Statue of Zeus in Olympia or the Temple of Artemis in Ephesus) importance of the Wonder is immediately obvious. In others, the names themselves - such as The Colossus of Rhodes - are quite telling. When necessary, descriptions fill in the blanks: the Mausoleum at Halicarnassus is 45 meters in height, and it's adorned with 400 freestanding sculptures. Finally, imagination makes up for whatever the name or description might be lacking as it seems to be the case with the Hanging Gardens of Babylon.
The Pyramids, like the Mausoleum, are huge - and they served an essentially identical purpose. While the Mausoleum was built for (and named after) a 4th century BC local king called Mausolus, the word itself morphed to take on a new meaning, that of “any above-ground tomb”. That's exactly what the Pyramids were built for - to become tombs for Egyptian Pharaohs, the most powerful rulers of their time. Hence the significance and the resulting grandeur of those structures whose size and appearance had to match the lofty status of their residents. Any discrepancy in that respect would inevitably affect the quality of the rulers' afterlife - or so the subjects of the Pharaohs believed.
The belief in question wasn't restricted to Egypt or any particular geographical region. Left at Nature's mercy and facing the horrors of the Unknown, all primeval societies would scramble to cope with such major mysteries as birth and death (not to mention some “lesser” challenges - the elements, various diseases and such). One of the major strategies of coping was reducing the unfathomable to something more or less familiar. The main product of that reduction was the myths of Afterlife - and all the places it could be spent in, such as the Gardens of Eden, the Realm of Hades, and their various equivalents in other cultures. Accordingly, numerous burial rituals were created by pre-scientific cultures to provide the deceased with everything necessary for a smooth transition. One of such cultures found its way to what's known in Ireland as “the Ancient East”.
Located in Boyne Valley, the “Irish Pyramids” - including Newgrange, Knowth and Dowth - are attributed to nomadic people that traveled 5000+ kilometers from halfway across the world before reaching the region in question. They seem to have been a nation of farmers that managed to not only develop building skills but also advance astronomical knowledge - so, when they finally settled down, they took their time (several generations or so) to create something truly outstanding. The structures they built were aligned with the movement of celestial bodies - and that resulted in what's known as “The Winter Solstice Effect” when a sunray travels through a tiny hole in the roof (the so-called “roof box”) and illuminates the otherwise utterly dark chamber for several minutes. That happens every year, on 5 days in a row (December 19-23), and the "show" starts at 8.58 in the morning. That almost supernatural precision alone shows the scope and magnitude of the “Irish Pyramids” wonder…