As an island, Ireland has a deep connection and relationship with the ocean and seas that surround it. Water has visibly and dramatically shaped the island physically and culturally for its entire history. The coastline is marked by towering cliffs, windswept beaches, abundant bays and thriving river estuaries. Some of the stormiest seas on the planet pound against the west coast, while the Irish and Celtic Seas have acted as conduits for ideas, migration, and invasion for centuries. The Atlantic Ocean also makes the country green – the gulf stream brings warm water that tempers the Irish climate; without it, the island would resemble Newfoundland, which lies on a similar latitude but experiences a far harsher climate. Water has brought wealth too. Trade has allowed port towns to flourish and grow, while fisheries have been the lifeblood of many coastal communities from prehistory to the present day. Ireland's fisheries of the fifteenth and sixteenth centuries are the focus of this book. Despite the importance of the marine space in Ireland's story, we know very little about the history of fisheries, and the subject has generated little interest among historians to date.

Ireland is not alone in this lack of knowledge; until recently, the history of the seas and oceans globally has remained largely unknown. There are many studies about things that float on water – ships carrying trade, migrants, and weapons – but few studies have been interested in what lies beneath the surface and how humans have interacted with it. Part of this may stem from older perceptions of the ocean as inexhaustible and unknowable. And while there remain many unknowns, we are very aware today that the oceans are far from inexhaustible and are, in fact, frighteningly fragile in the face of unchecked human exploitation and climate change. The fragility of marine life and ecosystems becomes only starker in the presence of an accurate historical perspective. For this reason, history is vital.

It was the marine scientist Daniel Pauly who first argued for the existence of a ‘shifting baselines’ syndrome, in 1995; he contended that a lack of memory about past ocean productivity often results in managers making decisions that keep fish populations depressed.