The woman, who is in her 70s, was attacked in Sneem, County Kerry, on 20 September.
The man was detained by officers in Killarney on Saturday morning.
He is being questioned at Killarney Garda (police) station.

Venera Minakhmetova died when she was struck by lorry at Bow Roundabout, on Wednesday.
The Russian, who had been on the Cycle Superhighway 2 (CS2) just before the collision, was the fourth of five cyclists to die in London in nine days.
Her sister Dina said: "If you know something, if you witnessed it, please come forward and contact police."
She also thanked people for their support following her sister's death.
Ms Minakhmetova had worked at London-based Edelau and investment bank Merrill Lynch and was living in the Bethnal Green area to develop her financial services business.
So far this year, 13 cyclists have been killed, eight of which have involved HGVs.
The leader of Southwark Council, Peter John, has called for HGVs to be banned during rush hour to protect cyclists.
Currently, the movement of HGVs are restricted in London between 21:00 and 0700 on weekdays, and between 13:00 on Saturdays through to 07:00 on Mondays.
Speaking to BBC Sunday Politics London Ben Plowden, of Transport for London, said the Olympics showed it was possible to achieve a very significant change in when and how deliveries take place.
"Something like a fifth of freight activity moved out of the busiest times in the day and that was because we worked closely with the boroughs and the councils, the freight industry and the supermarkets and other people to make that happen," he said.
"So what that shows is you can make a big change, the question is how do you make that part of everyday life rather than a one-off thing for the Olympics?"
Catherine West, head of transport on London Councils, said a balance was needed so that Londoners could get a good night's sleep.
"A lot of people live on busy roads and they live above shops. We just have to be mindful that a lorry delivery at 4am may not be great for residents," she said.

Rherras is a 23-year-old left-sided player of Moroccan descent who has spent the last year playing for Sint-Truiden in the Belgian top flight.
Sammon, the 29-year-old ex-Kilmarnock forward, has agreed a three-year contract as he leaves Derby County.
They will arrive as Hearts prepare for their first Europa League qualifier.
Six players left Tynecastle in the summer, with three first-team regulars - defender Blaze Augustyn, goalkeeper Neil Alexander and midfielder Miguel Pallardo - all now on the lookout for new clubs.

Researchers funded by the charity found that by looking at a specific group of genes they were able to reliably detect underlying problems.
It follows the death of Sir David Frost's son Miles, who died suddenly from a condition thought to have been inherited from his father.
His family aims to raise Â£1.5m to make genetic testing available in the UK.
The eldest son of the late presenter and broadcaster died last year, aged 31, when he was out jogging near his family's home in Oxfordshire.
He was suffering from an undiagnosed heart condition called hypertrophic cardiomyopathy.
Previous genetic tests looked at a small number of genes and were only able to identify specific conditions, leading to higher costs and longer diagnosis time - a major barrier to rolling the test out across the NHS.
But researchers from Imperial College London and the MRC Clinical Sciences Centre say the new test, which looks at 174 genes, is quicker and more reliable.
It has already been rolled out at the Royal Brompton and Harefield NHS Foundation Trust in London , with around 40 patients per month being successfully tested.
Dr James Ware, a consultant cardiologist specialising in inherited heart conditions, said: "Without a genetic test we often have to keep the whole family under regular surveillance for many years. This is hugely costly for both the families and the health system.
"By contrast, when a genetic test reveals the precise genetic abnormality causing the condition in one member of the family, it becomes simple to test other family members. Those who do not carry the faulty gene copy can be reassured and spared countless hospital visits."
The research has been funded by the British Heart Foundation and the findings published in the Journal of Cardiovascular Translational Research.
A man in his early 20s has been arrested by police investigating a serious sexual assault on a pensioner in the Republic of Ireland.
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The sister of a cyclist who was killed in east London last week has urged witnesses to contact police.
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Hearts are expected to confirm the signing of defender Faycal Rherras and striker Conor Sammon ahead of a return to pre-season training next week.
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A new blood test could help diagnose people with inherited heart conditions, the British Heart Foundation has said.