I-YRCH MELAITH DURUBGOR The Mighty Orcs of Durubgor

Here is how I came to this conclusion yrch is a the plural for orcs, melaith root form √MBELEK is the plural soft mutated form of belaith adjective for mighty. Under the Internal i-affection rule for non-final syllables the e stays the same, and the Final i-intrusion ai does not mutate. I did not use na again in this sentence because it should already be implied that they are the mighty orcs (of) Durubgor.

This is mostly correct. Just one thing: before plural nouns i is in.

I assume Durubgor is not mean to be Elvish?

Correct, Durubgor is a Orcish name. Taken form MERP Roleplaying manual for Orc Names.