Much has been written about St. Patrick, much of it marginally conflicting, and because he was around nearly 2,000 years ago, solid facts are impossible to verify. The following is probably as close to the truth as you will get, not that any discrepancies really matter that much, (in my opinion).
Patrick was not Irish. He was
born in Britain around AD 385. Captured in his youth by Irish pirates
raiding the Scottish coast, he was sold into slavery in Ireland.
Later, escaping to Europe, Patrick spent some years studying at the monastery of St. Martin of Tours in France, where he was ordained as a priest. Deeply affected by the Christian missionary zeal, so prevalent in the early fifth century, he decided to return to Ireland to undertake the conversion of the Celtic pagans and their Druid priests.
Arriving in Ireland in AD 432, Patrick spent nearly thirty years travelling about the countryside, bringing Christianity to the local people and establishing churches and monastic foundations upon many Druidic sacred sites, which had themselves been established upon far more ancient megalithic sites of the Grooved Ware people.
There is a very ancient route leading from Leacht Benain on Croagh Patrick to Ballintubber Abbey. This route is now known as St Patrick's Causeway and pre-dates Patrick’s visit to the Reek. Originally it probably stretched from Cruachan, near Boyle in Co. Roscommon, which was the ancient seat of the Kings of Connaught. All along this causeway, which runs in a straight line from east to west, many churches, abbeys and settlements were established, most of which are now in ruins.
Later, escaping to Europe, Patrick spent some years studying at the monastery of St. Martin of Tours in France, where he was ordained as a priest. Deeply affected by the Christian missionary zeal, so prevalent in the early fifth century, he decided to return to Ireland to undertake the conversion of the Celtic pagans and their Druid priests.
Arriving in Ireland in AD 432, Patrick spent nearly thirty years travelling about the countryside, bringing Christianity to the local people and establishing churches and monastic foundations upon many Druidic sacred sites, which had themselves been established upon far more ancient megalithic sites of the Grooved Ware people.
There is a very ancient route leading from Leacht Benain on Croagh Patrick to Ballintubber Abbey. This route is now known as St Patrick's Causeway and pre-dates Patrick’s visit to the Reek. Originally it probably stretched from Cruachan, near Boyle in Co. Roscommon, which was the ancient seat of the Kings of Connaught. All along this causeway, which runs in a straight line from east to west, many churches, abbeys and settlements were established, most of which are now in ruins.
It is thus logical to assume this was the route that Patrick used when
coming to this area. Starting from Ballintubber Abbey, through
Triangle, he reached Aughagower, where he stayed before ascending the
mountain.
Here he established a church, and a holy well - The Well of Deacons (Tobar na Deochaun) used to
baptise his converts to Christianity. In 441, Patrick and members of his
household, accompanied by St. Benan of
Kilbannon departed on Shrove Saturday to spend the days of Lent on
Cruaghan Aigle (Eagle Mountain), now Croagh Patrick. He fasted for forty days, and
reputedly banished dragons, snakes, and
demonic forces from the site. They returned on Holy Saturday to
celebrate Easter with Bishop Sinach in Aughagower.
The journey passed through Bohea, a town-land 6km to the east of Croagh Patrick where there is a remarkable rock outcrop, decorated with ancient art, and known locally as the Bohea Stone. It was discovered in recent years that on two days of the year, the 18th April and 24th August the setting sun directly hits the top of Croagh Patrick and instead of setting behind it, as it usually does, it slides down along its northern slope in a rolling movement.

It is also said that Patrick established a simple chapel on the summit
of Croagh Patrick called Teampall Phadraig. Existence of the chapel was
found on the summit in 1994. There is evidence of this chapel’s
existence recorded in 824 when the Archbishop of Armagh argued
with the Archbishop of Tuam, as Armagh was claiming church dues from
Teampall Phadraig. This shows that the church on the summit was in use.
The foundations of this church are still on the summit of ‘The Reek’.