Complex sentence: But navigation of the western seaboard of the island demanded intimate knowledge, and the experience of Spanish mariners in the intricacies of the Irish coast and weather conditions was largely confined to trading voyages to the south and south-west of the country.
Split into simpler sentences: But navigation of the western seaboard of the island demanded intimate knowledge. The experience of Spanish mariners in the intricacies of the Irish coast and weather conditions was largely confined to trading voyages to the south and south-west of the country.