I was just listening to this on the news. Perhaps City of Derry Airport was considered too close to the actual city of Derry so it was decided in keeping with Ryanairs policy of flying to remote airports in the a$$hole of nowhere, that this military base was more suitable

Regarding the announcement after landing, they interviewed one passenger who said that the captain simply said 'we appear to be at the wrong airport, buses are on there way to take you to your final destination'