The man, who is in his 80s, was assaulted at his home at Ballintra on the island.
A sum of money was taken and the victim was “manhandled” but not badly injured and the alarm was raised a short time later.
The man’s attacker was understood to be disguised in a certain type of clothing.
Garda Superintendent David Kelly said because there were no ferry sailings off the island after 8pm when the attack happened, there is a suspicion that the attacker may be from the island.
“That is something I am considering highly,” he said.
Gardaí made a ferry crossing to the island last night in a bid to launch an investigation into the alleged attack.
Gardaí will return to the island today carrying out a full investigation into the incident.
People on Arranmore Island say they are ‘shocked and appalled’ by the attack on an elderly man.
A spokesperson for the Arranmore Island Community Council issued a statement in which they criticised the attack on the pensioner, describing it as an “unprecedented “ incident.
“This crime is unprecedented and understandably the community are shocked and appalled.
“We will be pulling together as much as ever to ensure such an incident is a one off.
“We have a proud tradition of looking out for one another on Arranmore Island, especially the more vulnerable in our community, and such an incident will only go to strengthen that community ethos,” a statement from the community read.
Anybody with any information is asked to contact Milford Garda station on 074 915 3060.