As a general rule, pilgrims doing the Celtic or Irish Camino usually walk the first 25 - 30 km in Ireland, then travel to A' Coruña - typically by boat or plane. From there, they travel to the A' Coruña seaport, and turn to walk the remaining 75 or so km to Santiago. The document mentioned above form the Irish Pilgrim Society should contain all this information.
If you came by ferry, starting from the ferry dock is appropriate. Otherwise, you need to get to the port from the airport. You can obtain sello or stamps at most any business. They ALL have backup rubber stamps, required by law, to be used in case the electronic register goes down, and receipts have to be hand-issued.
Hope this helps.
Tom