Overview
Liam is a short form of the Irish name Uilliam or the old Germanic name William.
Etymology
The original name was a merging of two Old German elements: willa ("will" or "resolution"); and helma ("helmet"). The juxtaposition of these elements effectively means "helmet of will" or "guardian".
When the Frankish Empire was divided, the name developed differently in each region. In Northern Francia, Willahelm developed first into "Willelm" and then into "Willaume" in Norman and Picard, and "Guillaume" in Ile-de-France French. The Norman form was further developed by the English into the familiar modern form "William".
Origin
Although the names Willahelm and Guillaume were well known in England before 1066, through Saxon dealings with Guillaume, Duc de Normandie, it was viewed as a "foreign" name. The Norman Conquest had a dramatic effect on English names. Many if not most Saxon names, such as Ethelred, died out under the massive influx of French ones. Since the Royal Court now rang with names such as Alain, Guy, Reginald and William, they were quickly adopted by the English, the Welsh, and eventually the Irish.
From Wikipedia (CC BY-SA 4.0).