La Unión is about 45 km east of San Miguel, which is the main transport hub for eastern El Salvador. Buses run regularly from San Miguel and continue toward smaller towns near the Honduran border.
From San Salvador, expect a long cross-country trip via San Miguel. Roads are better than they used to be, but this is still the far eastern end of the country. Boats to the gulf islands are arranged locally from the waterfront and depend on weather and demand.