This is reachable on a road bike but not really advised. The approach is gravel and the breakwater itself is very exposed, with frequent strong winds and waves breaking over the sea wall. Can be sketchy in anything other than calm conditions.
Holyhead Breakwater is situated at the north-western end of Holyhead in Anglesey. The Victorian structure, which is 1.7 miles (2.7 km) long, is the longest breakwater in the United Kingdom. The breakwater, which is accessible in good weather, has a promenade on top which leads out to the Holyhead Breakwater Lighthouse. It can be very slippy where the water comes around and moss has grown. Not ideal to ride over in bad weather.