First, the treacherous coasts around Redstone Island, due to its volcanic geography, are more dangerous than most nautical threats. The rocks rise up quickly and are sharp as razors, able to tear out the hull of a ship in a few moments. Historically, scavengers would settle around shipwreck areas to pick up lost cargo. Now imagine that in a fantasy setting with sea monsters and other threats looking for a tasty meal.
Second, in relation to the region, even though the coastline is a coupe hundred miles long, the island is very small and easy to miss as it sits by itself in the Erse Sea. Having multiple lighthouses is crucial to the town getting any trade at all. Miss the island because you can't find it and your next port of call is ten days away, if you make it at all, as you also need to skirt the dangerous Sunken Lands. With no functional lighthouses right now, that means trade is very light to the island as the threat is high.
Third, the island was "settled" by the Northmen, who came from the, you guessed it, north. They did not need a lot of lighthouses as their journey was shorter and less dangerous. Now most of them live far to the east, past the Sunken Lands and they face the same navigational threats as everyone else.
If you want to improve trade, you should definitely repair the various lighthouses, which use natural crystals infused with magic (continual light) to guide ships (you have two such crystals). A lighthouse at the Tara harbor would further increase trade and would be much appreciated.
How to Navigate: The approximate Earth year is zero. This means sailors tended to use the stars and moon to navigate, and most stayed along the coastline using landmarks or even scent (cities smell). There were no compasses or other navigation tools and even maps were rare. The Northmen, with heavy cloud cover many months of the year, could watch the birds. In fact, most voyages were very repetitive. Rather than the oceans as a superhighway, a medieval development, many sailors made the voyage between the same two towns with the same cargo for their entire lives.
"When you get 11 fathoms and ooze on the lead, you are a day's journey out from Alexandria," wrote Herodotus in the fourth century B.C.