1879-80 eruption

The last eruption of Ilopango constructed a new lava dome which emerged above the caldera lake and formed the Islas Quemadas, the "burnt islands".
It has been speculated that the eruption was affected by solid-earth tides, but a much more convincing and likely explanation was provided by Richer et al (2004) who demonstrate that an injection of 2% of basaltic-andesitic magma at the base of partially crystallized dacite magma triggered the 1880 eruption, creating the necessary overpressure needed for an eruption.

5th century eruption of Ilopango (around 450 AD)

A violent Plinian eruption occurred at Ilopango in the 5th century AD. It was the largest eruption in El Salvador during the past 10,000 years and one of the largest in Central America.
The eruption was larger than the Krakatau 1883 or Pinatubo in 1991, and probably more comparable to Tambora 1815. It ranks VEI 6-7. It erupted more than 70 cu km of tephra, produced major ash fall and large pyroclastic flows that covered 10,000 sq km under 50 cm or more of pumice and ash. Pyroclastic flows and ash fall devastated an area of up to 100 km radius around the volcano.
The eruption severely impacted the cultural history of the region: at the time, a sophisticated Mayan culture ruled and lived in the highlands of El Salvador. The eruption killed thousands, destroyed settlements and sent many to flee to the lowland areas in Guatemala and Belize. It ended the Mayan presence in the highlands, and it took probably several centuries for the region to recover.
A major trade route controlled by the Mayas was abandoned and power shifted to Tikal. Modern excavations of some of the buried settlements have been bringing light into Mayan culture.