They do look very similar until you get really close up.

The undifferentiated passage tombs in Waterford are actually very close in design to the Scilly Isles passage tombs (see TMA book for those). Even the finds in them are identical.

The Hunebed are mainly staright sided, but the sides of the Waterford tombs go parallel for a while and then flare out slightly to form the passage. Hence "undifferentiated passage tomb". The Hunebed are actually closer to the 'blockier' wedge tombs (see Lough Gur wedge tomb in Co. Limerick or Proleek wedge tomb in Louth)

The Waterford tombs also have a kerb (well, except Matthewstown - typical). Have a look at Harristown in Co. Waterford (too lazy to give link).

There is one other like it at Carriglong (Co Waterford), but on my last visit the crops were too high.