The man is currently being detained at Enniscorthy Garda Station.
A garda spokesperson said they are continuing to investigate all the circumstances of the incident.
“As a result of an investigation into subsequent related matters, a man in his 20s was arrested under the provisions of the Non-Fatal Offences Against the Person Act, 1997,” gardaí confirmed.
"He is currently being detained under the provisions of Section 4 of the Criminal Justice Act, 1984.”
Nine-year-old Alejandro Miszan was playing football with his friends on the green of the Old Forge Road estate in Enniscorthy on Sunday afternoon when he was set upon by a pit bull.
He is currently in Crumlin children’s hospital in a serious condition.
His older brother, Raul (21), told Independent.ie that his younger brother now has no lower lip left, cannot eat or breathe by himself and a tube has had to be inserted into his neck.
He has had to have a number of surgeries carried out on his face due to the extent of his injuries.
Sergeant Stephen Ennis, who is based in Wexford, told South East Radio that gardaí will be doing house-to-house enquiries over the next day or two as they appeal for witnesses to the attack.
“Our thoughts and prayers are with the young lad with his recovery which is no doubt going to be difficult,” he said.
Following the attack, two dogs have been seized by Wexford County Council.
A spokesperson for the council said it is liaising with gardaí in relation to the investigation into the incident.
In a statement, the council said: “Following a very serious attack on a young boy by a pit bull cross [now deceased] in Enniscorthy on Sunday, two other dogs are now in the possession of Wexford County Council.
"The dogs will be assessed by the county veterinary officer. The concern of Wexford County Council is for the welfare of the young boy injured and his recovery.
Politicians have called for a ban on certain breeds of dogs following the horrific attack on young Aljandro.
Fianna Fáil senator Malcolm Byrne said he fears it is “only a matter of time before a child loses his or her life” after a spate of attacks involving pit bull dogs this year.
Speaking in the Seanad about the “life-changing” injuries suffered by Alejandro after he was attacked by a dog near his home in the Old Forge Road estate in Enniscorthy on Sunday, Mr Byrne said the current regulations in place are “not strong enough”.
In Ireland, laws introduced in 1998 imposed restrictions on 11 different breeds of dog including Rottweilers, German shepherds (Alsatians), American pit bull terriers, English bull terriers, Staffordshire bull terriers, bull mastiffs, Dobermann pinschers, Rhodesian ridgebacks, Japanese akitas, Japanese Tosas and any dogs which are a cross of those on the list.
Included in the restrictions are that the animals be muzzled while in public and held on a strong lead by a person above the age of 16.
Under the Control of Dogs Act, fines of up to €2,500 and imprisonment for up to three months can be imposed on owners of marauding dogs.
“A question has to be asked, should we continue to allow some of these dogs to be allowed to be licensed at all?” Mr Byrne said.
“I fear it is only a matter of time before a child loses his or her life in one of these attacks.
“I think we need to take action. I’m asking that we could look at a review of the Control of Dogs regulations and it is my view that for these particular 11 breeds, it is time to consider a ban or stronger regulation,” he said.
Senator Aidan Davitt said unmuzzled dogs are a “common sight” in parks in Ireland.
He called for stronger laws and punishments to be introduced for owners who do not abide by the laws currently in place.
“To have animals like this running loose is not acceptable,” he said.
“The law is very specific, clear and easy to understand.”
A Go Fund Me account set up to raise funds for the Miszan family has so far raised €52,000.
Raul said his parents are “heartbroken” and thanked the people who came to his brother’s rescue.
He said one man used a baseball bat to fend the dog off.
Raul is now calling for owners to “take responsibility” for their dogs
“Who will pay for my mother’s tears, my father’s tears? What about Alejandro? The dog died. What if my brother dies? I like animals and I am not trying to be mean but just saying.
“My brother’s face is in bits. He doesn’t have a lip, it’s completely in bits. His legs, his tummy, it is all in bits. Who will pay for all this?
“My parents are devastated. It is a disaster. We are afraid he won’t feel the same about himself. He doesn’t know how he looks now and we are afraid about when he will see himself.
“I want justice. I don’t know where this will go but hopefully the guards will do their job.”
Gardaí confirmed their enquiries into the attack are ongoing.
Earlier this year a child had to be rushed to hospital after a dog attack in south Dublin.
The young boy was attacked in his own home by a dog believed to be a cross between a pit bull and a Presa Canario.
The city council had to call in a lion expert after fears the animal would be too dangerous to handle and a tranquiliser gun was used to sedate the animal.
The dog was later put down.
In Tipperary, a young woman was hospitalised for a number of days following an attack in Carrick-on-Suir in February.
She was out walking her two dogs when they were attacked by what are believed to have been pit bull terriers. The woman underwent a number of surgical procedures.
The public can donate to a Go Fund Me for Alejandro Miszan