For background on how the map was made (and its flaws), see the relevant blog entry.
|MacWilliams||numerous: Ulster generally, Dublin etc. Ir. Mac Uilliam (Liam). The first name William, Teutonic Willhelm, meaning "will-helmet", was common amongst the Normans. See also Williams. SI.|
|Quilliam||Very rare: Belfast. Ir. Mac Uiliam. May be of Manx origin.|
|Williams||Very numerous: all areas. MacLysaght describes it as Welsh, but it is common all over Britain. A Norman first name, adopted from Teutonic Willihelm "resolve-helmet". It has produced many variants. See Mac Williams. Ir. Mac Liam. DBS & DOS.|
|Williamson||Very numerous: mainly Ulster where they out-number Williams 2-1. Also Dublin, Cork and Midlands. Believed to be mainly of Scottish origin. See Williams.|