Many people who head for Anglesey (myself included the first time, I'm sorry to say) cross over from the mainland and whizz right across the island to Holyhead, from where there is a regular ferry service to Ireland, with crossings to Dublin and Dun Laoghaire. The crossing is short enough to make a day trip to Dublin a possibility. But for those who decide to linger on Anglesey there is plenty to see in the vicinity of Holyhead. One thing to note, however, is that Holyhead is, strictly speaking not actually on Anglesey, but on Holy Island, a smaller island which is reached from Anglesey via the A55/A5, or alternatively a more minor road crossing at Four Mile Bridge, so named not because of its length, but because of the distance from Holyhead. There was a Roman fort in Holyhead in the 4th century AD which was used as a coastal defence from Irish raiders, and the original walls can still be seen, surrounding the 13th century Church of St Cybi. One Irish raider called Seregri is buried in the churchyard, his grave marked by the small chapel of Eglwys-y-Bedd.
More ancient sites can be found on the summit of nearby Holyhead Mountain, where there are remains of the Iron Age fort of Caer y Twr, incorporating the ruins of a 4th century Roman beacon. Lower down is the 3rd century settlement of Cytiau'r Gwyddelod, with the walls of 20 huts surviving. To the west of Holyhead Mountain is the South Stack RSPB nature reserve where you can park up and wander down to the crenellated Ellen's Tower, which acts as the Visitor Centre. Alternatively, for a fee, you can descend the 400-odd steps leading down to a footbridge which gives access to the small island - or large rock, depending on how you view it - of South Stack itself, topped by a 90-foot high lighthouse. The cliffs around here are colonised by large numbers of seabirds, including a colony of puffins. The day we visited was a beautiful sunny day, and we had a wonderful alfresco lunch outside the reserve's cafe, gazing out at an alluringly azure Irish Sea.
Map of the area.