17Horn, Walter, Jenny White Marshall, and Grellan D. Rourke. The Forgotten Hermitage of Skellig Michael. Berkeley:  University of California Press,  1990. 79-80.
The mentions of Viking raids are found in the text "War of the Gaedhil with the Gaill," the first in an entry of 821–823 is the one about Etgal, the other (undated) occurs after the Etgal entry and before another dated 850.
Bourke, Hayden, and Lynch offer further details regarding the second Viking raid: "A single Norse raid is recorded in annals, in 824, resulting in the death of Étgal, possibly the abbot but certainly an important cleric. A record of a further raid is preserved in the Cogad Gáedel re Gallaib (CGG). CGG is a problematical text, as it is primarily a work of political propaganda (though early scholars mistakenly viewed it as a historical treatise), but it does draw directly on the contemporary annalistic records (Ní Mhaonaigh 1996), many of which do not survive to the present. So it is likely that the reference to the additional raid on Skellig Michael is authentic." (Bourke, Edward, Alan R. Hayden, Ann Lynch. Skellig Michael, Co. Kerry: the monastery and South Peak: Archaeological stratigraphic report: excavations 1986–2010. Rep. Dublin: Department of Environment, Heritage and Local Government: Office of Public Works, 2008. 21-22.)