What a lovely quaint term that is! Along with the quoits and Welsh tombs of similar ilk, 'entrance graves' are derivatives of the Irish portal tomb.

For a whopping 'earthfast dolmen' see:

http://www.themodernantiquarian.com/browse.php?site_id=887

The terms 'entrance grave' was used for all portal tombs up until about 20 years ago when 'portal tomb' was adopted for them. As mentioned above, the 'earthfast dolmen' is now recognised as simply being a sub-type.