DNA evidence


DNA stands for deoxyribunucleic acid. DNA is the blueprint for your genetic makeup. It consists of a pattern of bands, a bit like a barcode. The way that DNA bands are arranged is unique to each person. There are some common features between the DNA patterns of parents, their children and other relatives.

DNA testing

DNA testing can be done on a tiny sample of biological matter, for example, blood, semen or saliva. When a DNA test is done, a profile of the DNA is taken. DNA testing can be useful evidence and can be used to:

  • Establish paternity: for example, you can find out if you are the parent or child of another person by comparing your DNA profiles.
  • Prove identity: for example, if a sample of blood is found at the scene of a crime, you can do a DNA test on the blood and compare it to a suspect's DNA sample. Then you can confirm if the blood found at the crime scene is the suspect's blood.

DNA evidence in court

DNA evidence is always explained by an expert witness when it is introduced in court. The expert tells the court how often that DNA profile is found in the general population. For example, the expert may tell the court that this DNA profile is found in approximately 1 in every 1,000,000 people.

The DNA database

Under the Criminal Justice (Forensic Evidence and DNA Database System) Act 2014 a DNA database was established in Ireland. This database is used:

  • As an intelligence source for criminal investigations
  • To help find missing people
  • To help identify unknown people

The Act states that certain DNA samples can be taken to be added to the DNA database. Forensic Science Ireland manage the DNA database. The database has an investigation section and an identification section. The investigation section is used to investigate suspected crimes and the identification section is used to help identify unknown and missing people.

The investigation section has a number of indexes including:

  • A reference index: which contains the DNA profiles of people who gave DNA samples under the 2014 Act, and people who gave samples under the repealed Criminal Justice (Forensic Evidence) Act 1990
  • A crime scene index: which contains samples taken from crime scenes
  • 3 elimination indexes: which are used to eliminate certain people’s DNA from an investigation. One elimination index has DNA samples from staff of Forensic Science Ireland, another has samples from Gardaí, and the last has samples from prescribed persons. Prescribed persons are other people who may have contaminated DNA samples in the course of their work.

DNA database and other EU member states

The 2014 Act provides for increased co-operation with other EU member states in relation to information on DNA databases. This means that one EU member state can ask another EU member state to perform a DNA database search on their system to see if they have a match for a particular sample.

What is the DNA database used for?

The database can only be used for its intended purposes, for example, only certain indexes can be compared in certain circumstances.

Your DNA profile will only be logged on the database if:

  • You are detained for a relevant offence. A relevant offence is one that has a maximum sentence of 5 years or more (see Section 9 of the 2014 Act).
  • You are an offender or a former offender. An offender in this case is someone who is serving a sentence for a relevant offence, while a former offender is someone who served a sentence for a relevant offence and has been released in the last 10 years.
  • You are a volunteer. A volunteer may be someone else involved in a case, for example, the victim.
  • Your DNA profile was generated under the 1990 Act. These DNA profiles were added to the database, unless they were due to be destroyed.

DNA samples

DNA samples are divided into database samples and evidential samples. A database sample is taken so it can be added to the DNA database and an evidential sample is taken to investigate a crime, but in most cases it is also added to the database.

Database samples are taken from a sample of hair (other than pubic hair) or from a swab of the person’s mouth. Evidential samples can be intimate or non-intimate samples. Intimate samples include blood, pubic hair and urine, while non-intimate samples can be saliva, hair or nails. When you are asked to give a DNA sample you must be informed if it will be added to the DNA database.

A database sample can be taken from someone who is arrested and detained for a relevant offence (an offence that has a maximum sentence of 5 years or more, see Section 9 of the 2014 Act). In these cases the database sample can be taken without your consent, and reasonable force can be used to get the sample. You must be told a number of things before the sample is taken, including that your DNA profile will be added to the DNA database.

In some cases a volunteer will be asked to give a DNA sample. Volunteers may be someone else involved in a case, for example, the victim. DNA samples can only be taken from volunteers to investigate specific offences. The volunteer must get extensive information about the process and must give their consent to have the sample added to the DNA database.

Exemptions and protections for certain people

Database samples cannot be taken from certain people. These people are called protected persons and include:

  • People who don’t have the capacity to understand the DNA testing process and cannot give consent
  • Children under the age of 14

Protected persons do not have to give database samples, but must give evidential samples, if required. When a protected person gives an evidential sample, information about the process must be explained to them in an understandable way. If the protected person’s guardian refuses to give consent for the sample to be taken, the Gardaí can apply to the District Court to get the sample. A responsible adult should be present if reasonable force needs to be used to take a sample from a protected person. The Gardaí cannot use reasonable force to get the sample from a child under 12 years old.

Mass DNA screenings

Large groups of people can be DNA tested to help investigate an offence. But these samples cannot be entered onto the DNA database.

Further information

You should get legal advice for more detailed information on this.

Page edited: 28 June 2022