The traditional interpretation of the motto is “The Gentle Hand to Victory”. Alternative interpretations include “The Noble Hand to Victory”. While “lamh” is a recognizable Irish word for hand, “foistenach” is more obscure. “Abu” is a common word in Irish clan mottoes.
Etymology of the Name: O'Sullivan 
The traditional interpretation of the motto is “The Gentle Hand to Victory”. Alternative interpretations include “The Noble Hand to Victory”. While “lamh” is a recognizable Irish word for hand, “foistenach” is more obscure. “Abu” is a common word in Irish clan mottoes.
its actually “An Lamh Foistenach Abu” and it means “The Steady Hand to Victory” you would be surprised how many people get it wrong.
An alternative translation would be “The Steady Hand to Victory”.
The history and usage of the word “foistenach” is wanting. Are there any experts in Irish with insights?