For the surname Williamson in Ireland, there are 2,512 B rcds dating from 1697 and 2,183 M rcds dating from 1698. The relatively small number of rcds and its sound suggests it was transplanted from England/Scotland in the 16th or 17th century.
I mentioned the database I research. While it isn't complete and has some problems, it is arguably the easiest online source for Irish genealogical information. The only better source would be a 16 CD set from the LDS Family History Center. I think this set is expensive.
To my knowledge, there is NO keeper of the Williamson lineage. If you get into serious genealogical research, you might want to subscribe to Ancestry.com. This website is LDS related. This sect has a fetish for tracing its members' pedigrees and has been highly active and successful since the mid 19th century. Subscribing to it will likely get you access to some of the really indepth research.
Another source is Burke's Peerage. This source is particularly useful if you happen to have nobility in your lineage. Through the efforts of several generations of genealogists, Burke's, and other sources, my family has traced our origins back to the yr 639.