The hospital say he was seeking shelter and was not waiting to be seen when he died.
Man found dead in chair at Tallaght University Hospital 'had been unnoticed for 12 hours', family claim
The man found dead in a chair at Tallaght University Hospital had been unnoticed for 12 hours, his heartbroken family have claimed - writes dublinlive.ie
Tragic Brian Hamilton was discovered slumped in a chair and unresponsive by staff in the hospital's A&E on Saturday afternoon.
The 40-year-old was discovered dead around 4pm but his family claim CCTV footage shows him sitting in the same spot since 4am - 12 hours earlier.
His sister Paula toldIndependent.ie: "We just cannot understand how he could have been left so long. Surely there should be some protocol to check on people?
"We are all still only getting over the death of our mother in May last year, something which affected Brian badly, and now we are trying to get our heads around this."
Gardai said in a statement: "Gardai are investigating the death of a man in his 40s in the A&E department of Tallaght Hospital on Saturday afternoon.
“Gardai were alerted at 5.30pm. A Post Mortem will be carried out to establish the cause of death. However foul play is not suspected."
A statement issued by Tallaght University Hospital said the man had been seeking shelter in the hospital and was not waiting to be seen.
They said that they are co-operating with gardai and an internal investigation will be launched into the incident.
Read more news of Dublin on our site.