The city was moved. The old structures like the church were moved brick by brick and put back together in the new location.
From wiki: In the 1960s the Miño River was dammed to create the Belesar reservoir, putting the old village of Portomarín under water. The most historic buildings of the town were moved brick by brick and reconstructed in the new town, including its castle-style main church: Church of San Juan of Portomarín.
