As an Ulster Protestant, and a determined opponent of Catholic Emancipation, he incurred the bitter enmity of Daniel O'Connell, who called him "the mortal foe", and worked for years to have him removed from office.
The Saurins were of French Huguenot descent (no doubt this is why Daniel O'Connell called William an "insolent transplanted Frenchman").
[2] He was educated at Duboudien's School, a well-regarded private academy in Lisburn,[2] and at Trinity College Dublin, where he took his Bachelor of Arts degree in 1777.
[2] Others noted that by "the nation" Saurin meant the Protestant ruling class with whose interests he wholly identified: never, according to his critics, did he admit that the Catholic majority had any rights at all.
That he was appointed Attorney-General for Ireland despite his opposition to the Union and his repeated refusal to stand for the post-Union Parliament is a tribute to his legal eminence (although Daniel O'Connell inevitably accused him of political time-serving).
His tenure as Attorney-General is remarkable not only for its length, but also for his effective dominance of the Irish administration, a position never equalled by any other holder of the office (with the possible exception of Philip Tisdall, Attorney General 1760–1777).
Saurin's effective control of the Dublin administration was well known to, and for many years taken for granted by the British Government; in time however, his inflexible opinions and unpopularity made him a political liability.
The publication of a letter he wrote to John Toler, the Chief Justice of the Irish Common Pleas, urging him to use his influence with Protestant juries to secure the conviction of Catholics in political cases, irrespective of whether they were innocent or guilty, did great damage to both of their reputations.
He was described as small and decidedly "French" in appearance (hence O'Connell's gibe about his being a "transplanted Frenchman"); his face was dominated by shaggy eyebrows, under which his black eyes had a piercing but not unkindly expression.