I can't find anything about emergency travel documents in their Ts&Cs, but I've found this relevant exchange from Ryanair's official twitter account:
Ann @ann_nguyen1 16 Jul 2015
@Ryanair I only have my emergency travel document and I'm flying on Sat from Manch home to Norway. Can I still board the flight?Pls say YES
Ryanair @Ryanair 16 Jul 2015
@ann_nguyen1 Hi, Can you advise who issued the document? TB
Ann @ann_nguyen1 16 Jul 2015
@Ryanair I just need to get home on Saturday! I have all the valid IDs with me. Emergency Travel Documents and my Norwegian ID.
RyanairVerified account @Ryanair
@ann_nguyen1 Emergency Travel Document issued by the Embassy is a valid travel document. GK
3:10 AM - 16 Jul 2015
Ann @ann_nguyen1 16 Jul 2015
@Ryanair thanks so much for the prompt reply!!! <3
Ann @ann_nguyen1 16 Jul 2015
@Ryanair one last Q. How do I check-in online when I don't have my passport?
Ryanair @Ryanair 16 Jul 2015
@ann_nguyen1 If there's a number on the travel document, put it as passport or check in at the airport (free of charge in this case). GK
Ann @ann_nguyen1 16 Jul 2015
@Ryanair also, would you be able to email me a confirmation of this? That I am OK to travel with an ETD.
Ryanair @Ryanair 16 Jul 2015
@ann_nguyen1 Emergency Travel Document is clasified as passport replacement and therefore no need for additional written confirmations. GK
https://twitter.com/ryanair/status/621622832888442880?lang=en
I'd suggest you contact Ryanair Customer Services again and explain that you have an Emergency Travel Document which is classified by Ryanair as a passport replacement and you'd like them to put a note on your account confirming for the airport staff that this is a valid document for travel.