Check out enya.com and the forum there (do a search for "synthesizer") and you get a lead to the following interview about Watermark: Enya Watermark Interview
Enya's synthesizers include the Roland D-50 (she likes the "heavy feel," optimum for playing sampled tympani and strings), the Fairlight III, the Yamaha TX (the rack version of the DX), an older Oberheim rack version, and the Roland Juno 60 ("We wouldn't part with it for anything in the world"). "We also used this really old keyboard called the Wave, which has got wonderful sounds, like the little sound on the beginning of 'Evening Falls,' and the quiet 'turn it up' part of 'Orinoco Flow.'" Enya plays a piano of unknown provenance, given to Roma by her auntie. They'd like to use the new MIDI-able Yamaha acoustic piano. Enya and Ryan use the Akai S-900 for sampling.