Any given Dungeons and Dragons campaign will have lots of NPCs to deal with, and many of them don't have names at all. Every now and then, an NPC suddenly becomes important enough that the party will need to referr to him once or twice, and a name becomes necessary.
Most likely, the time it takes to browse through a list of names from ancient whenever just wont be worth it. For cases like these, Dungeons and Dragons name generators are perfect!
There are uncountable Dungeons and dragons Name generators available on the internet, although they're not all for Dungeons and Dragons specifically. (Of course, neither are ancient Irish names, are they?) Currently, Brighthub.com has got your back if you need a name for almost any Dungeons and Dragons character, whether it's a Dwarf, an Elf, a Hobbit (or Halfling!) an Orc, a vampire, or even a pirate!
Name generators outside of the realm of Dungeons and Dragons can help sometimes, depending on the languages they offer.
This one at Behindthename.com allows you to select things like "Ancient" and some ancient mythologies.
Other categories that can work are German or Welsh. Consider finding an English word, and translating it to one of those languages. Sometimes you'll come up with suitable results that actually mean something.
If those don't do it for you, games like World of Warcraft usually have a random name generator as part of their character creation process. Another option is to use the built in name generators that most D&D character creators have.