Porth Penrhyn Mawr is a rather untidy beach of sand and shingle with some rocks, backed by banks of boulder clay. It's approximately 500 yards wide and looks out towards Holyhead. The lower part of the beach is mostly seaweed strewn rocks.
The road from Llanfwrog to Penrhyn Bay camping site passes the northern end of the beach, and access to the shore is down a short track. Dogs are allowed at all times. The water here is not as clear as on most Anglesey beaches. Large waves are to be expected following ferry movements at Holyhead port.