Colin, it’s just bad typesetting by the advertiser. All the Irish locos are prefaced by bold type, thus CIE. Unfortunately, BR classes are freely mixed in the list. This is simply a planned UK ‘superdeltic’ - class 51 - which never came to pass. The Irish kit was an E pure and simple.
.https://en.m.wikipedia.org/wiki/List_of_British_Rail_unbuilt_locomotive_classes