Constables at Common Law
Anglo-Saxon Background
The private constable developed from the tithing-man which was part of the Anglo-Saxons’ system of policing. Under the the laws of Canute, every free man over the age of 12 had to be a member of a tithing.
If a member of the tithing committed a crime, the others had to produce him to court and pay the fine incurred if the individual could not pay it. The tithing was subject to a fine and forfeiture if the guilty man was not produced. The tithing enjoyed customary powers to ensure its ability to undertake these policing functions.
Each tithing elected one member as a head man to represent it at the local assemblies, including courts which conducted both judicial and administrative business. The tithing-man had certain customary powers and privileges.
Powers
He could prevent or terminate breaches of peace. He could demand sureties from persons breaching the peace. He could confine persons in the stocks until the surety was for common staff. He could take steps against suspected wrongdoers on the basis of reports of a credible person. If could take surety from suspects and wrongdoers. He had immunity from liability for injury or killing a person who resisted his authority. He could call out the tithing to pursue the wrongdoers.
The powers of the tithing men derived from custom. He discharged functions which were customarily vested in the community. These functions derived from common law rather than from statutes. He acquired authority from the community.
Later Development
Historically, constables were royal military officials in Norman times.  By the 13th century, ordinances for the preservation of the peace provided for the appointment of constables in each village, and a chief constable in every hundred (historical local government areas.) Men sworn to arms  were obliged to assemble before the chief constable as required and follow his directions to take necessary measures to preserve the peace.
Each chief constable was subject to the authority of the sheriff and knights appointed for peacekeeping purposes. Â This mid-13th century legislation recognised constables who had been in existence since the earlier Norman times (1066-1154) in England.
In the 14th century, the justice of the peace replaced the sheriff in relation to much local administration. Â The constable came to be a ministerial officer of the crown and gradually replaced the tithing-man in peacekeeping duties. Â The tithing-man doubled as constable in many places until the 16th century in England.
Legislation confers the status of a constable or peace officer and a person it attracts the common law privileges powers and duties.
Modern Constabulary
The Constabulary (Ireland) Â Act 1836 provided that the Irish constabulary, provided that the constables and sub-constables were to have the powers, privileges, duties and responsibilities conferred by common law or statute. The Irish constabulary became known as the Royal Irish Constabulary.
The Inspector General of the Royal Irish Constabulary was charged with the general direction of superintendence of the force. Â He had powers to make rules for the general government of the force, subject to the consent of the Lord Lieutenant.
An Garda Siochana
The Police Forces Amalgamation Act 1925 provides that all pre-independence references to the Royal Irish Constabulary in statute are to be interpreted as references to the Garda Siochana and references to the ranks of the inspector, inspector or sergeants and constable, or other officer or men are to be taken as reference to the superintendent, inspector, sergeants, garda or other officers or men of equivalent rank.
Although the legislation does not specifically refer to the common law office of constable, it  created an office which was intended to be the equivalent of the predecessor. Officers are appointed by the Commissioner rather than employed. It is a role rather than just an employment relationship They hold an office equivalent to that of constable. All powers and functions of the role are determined by legislation, rather than by a contract of employment.
The Garda Siochana is established as a force of police. Â The Garda Siochana itself is not a corporate entity. Â It is a body of officers under the general control of the (Garda) Commissioner.
Status of Garda at Common Law
Technically, all members of an Garda Siochana are peace officers, apart from their rank. Certain privileges, rights and responsibilities attached to the common law office of peace officer.
In English practice, a constable police officer was not an employee. Â His superiors are not, vicariously responsible for his actions. Â The principle is that the police officer exercises an original authority which is not one delegated by his superiors. Â They are set forth in statute and in common law. Â Their office has a function which owes duties to the State as a whole.
The constable in common law is independent in the exercise of his functions. Although independent in the exercise of his functions, a member of an Garda Siochana is of course subject to the authority of his superiors, provided they are lawful orders. Nonetheless, Â he retains authority regarding the exercise of his law enforcement powers. Â In many cases, he must exercise powers independently, by statute or at common law.