What is happening Philippa could explain better, but is called eclipsis. In this case the n is inserted in front of the d. this always happens after "ar an", translating to "on the". You see it again in the phrase ar a' tsráid, where the s is eclipsed by thr t. I think nDeor is the genetive case of Deor, but thaat's probably getting beyond me. Of the consonants that are eclipsed in Irish, c is eclipsed by g, b is eclipsed by m, s is eclipsed by t, d is eclipsed by n, f is eclipsed by bh, g is eclipsed by n, t is eclised by d. L,m, n, p and r are not eclipsed. I probably made it more complicated than it is.

Brían