The minister wrote: "Further to your recent contact with Peter O'Brien, our local election candidate in the Dun Laoghaire area, concerning voter registration."
She added: "I have enclosed the necessary forms that you will need to fill in and get stamped at your local Garda Station."
Ms Hanafin concluded: "If there is anything further that I can do to assist you, please do not hesitate to contact my constituency office."
The letter is thought to have been sent in error to an address in Blackrock, which is not in Mr O'Brien's or Mr Devlin's electoral area. But supporters of Mr Devlin have wondered how many more of the communiques have been posted, reaching homes in Dun Laoghaire.
A spokeswoman for the minister has insisted the letter was sent in the course of "normal constituency" work. She pointed out it was not sent out on Oireachtas headed paper and was responding to inquiries.
But the recipient has revealed she never contacted the minister or Mr O'Brien, who is Minister Hanafin's parliamentary assistant, regarding any local issue.
She insisted the person to whom the correspondence was addressed has not lived at the house for decades.
A supporter of Mr Devlin, who is the sitting Fianna Fail councillor in Dun Laoghaire, told the Herald they are disappointed he was not mentioned.
"We're disappointed that the party ticket has not been promoted at this juncture," the canvasser said.
Mr O'Brien said the letter in question does not refer to his electoral area so no one there would have contacted him.
"My electoral ward is not in Blackrock. It's in Dun Laoghaire. It could be clerical [a clerical error] but it's certainly not me [who was in contact with the householder]," he added.
Dun Laoghaire is a hotly contested ward. Cllr Devlin is the only Fianna Fail councillor in the electoral area. He and Mr O'Brien will be trying to win seats against Fine Gael's John Bailey, Mary Mitchell O'Connor and Tom O'Higgins, Labour's Jane Dillon Byrne and the Green Party's Gene Feighery. Local campaigner Richard Boyd Barrett is also vying for a seat.