jökull is Icelandic for glacier. Glaciers do not erupt, volcanoes do.
The volcano underneath the glacier is called Eyjafjall. Note it’s Eyjafjall, not Eyjafjalla. Eyjafjalla is the genitive (possessive) plural form: Eyjafjallajökull literally means “eyja mountains’ (sic) glacier”. (Fjall is Icelandic for “mountain”, it is derived from the Old Norse, as is our English word “fell”. Eyja is Icelandic for “island”, it is derived from the Old Norse íeg as is our word “island” – which is why the “s” is not pronounced. “Ey” lives on in English placenames, such as Anglesey, Guernsey, Jersey and Sheppey.)
Talking about the Eyjafjallajökull eruption is not only inaccurate, it is also long-winded. Let’s be accurate and more concise and say “Eyjafjall”.