There's nothing much there aside from a ferry-port to Dublin. It's Welsh name, Caergybi is named after Saint Cybi's monastery, which itself was originally a Roman fort (hence the name). The Latin name for the fort is unknown but it became Caer Cybi in Welsh. The town of Holyhead/Caergybi grew around it. The English name refers to the "head of Holy Island" and the Welsh name refers to Saint Cybi's fort.
When the two words Caer and Cybi are joined the second word undergoes Soft Mutation (c to g) and so becomes Caergybi.