UPDATE:

Update: 

Police can confirm that four persons have been transported to the hospital by ambulance to be treated for smoke inhalation as a result of the fire incident at HMP Northward. There was no structural damage done to property and the facilities have now been secured.

Fire investigators have conducted preliminary crime scene assessment and a joint investigation between the CIFS and RCIPS has now commenced regarding the circumstances that caused the fire.

From RCIPS

The RCIPS and other emergency services are currently on scene in response to a fire at HMP Northward. A cordon has been put in place and members of the public are asked to avoid the area until otherwise advised.