Anyone know of a particularly good reference source for tracking down ancestry and name origins?

I was trying to track down the origin of my Irish ancestry through the McKendry name and actually found that it also is linked with Clan MacDonald and Clan Hederson. I know that at least my great great grandfather was born in Ireland, but I want to try and figure out if it definately originated in Scotland or not. That of course would give me a very distant link with Scotland, but a link none-the-less!