"I feel rejuvenated," said the surprisingly-perky 70-year-old, who was close to completing the 170 miles from Cobh in Co Cork to Dublin's city centre.
And he came the hard way too.
At times it was cold and windy and often dangerous, as cars flew past him on uncomfortably-tight bends.
Now, instead of being in Dublin's outer suburbs, the priest was already at Harold's Cross and would be in O'Connell Street in less than an hour.
The radio bulletins had charted his progress all morning and by now dozens of people had turned out in the wintry cold to walk with him.
When he set off from St Colman's Cathedral in the heart of the diocese of Cloyne nine days ago, a local reporter and a handful of bemused spectators waved him off.
Now as he strode over Dublin's O'Connell Bridge in glorious January sunshine, Fr Mernagh was accompanied by victims of clerical abuse, Travellers from Pavee Point and several garda outriders.
As he headed past Eason's and on to the GPO, well-wishers stepped out to greet him, to thank him and to express their solidarity for what he was trying to achieve.
Yes, he walked the walk to put pressure on Bishop John Magee to quit over his failure to protect victims of clerical abuse in Cloyne, but no, he wasn't just targeting one man.
The walk was also to try to atone for the failures of the church in general, and to provide a voice for all those affected by abuse.
"I was a sign of hope for the many people who were abused and a symbol of someone who is finally prepared to listen," said Fr Mernagh, who thanked God for the blessing of good weather for the long walk.
With him was Paddy Doyle, the author of the 'God Squad', who joined Fr Mernagh an hour earlier. As the pair moved along deep in conversation, a few people wondered aloud what all the fuss was.
"Who's yer man?" said one woman at the Spire.
"I think he's off the telly," came the confident reply from her friend.
The publicity may not have reached everyone and Bishop Magee remains aloof, but yesterday those who really mattered were listening.
For waiting on the steps of the Pro-Cathedral for the arrival of the Augustinian priest was the powerful figure of Archbishop Diarmuid Martin.
Aching in every fibre and sinew after the long, hard miles, the ever-so-slightly stooped figure of Fr Mernagh turned into Cathedral Street at almost exactly 1.30pm.
Friends and supporters were there and so were the media. Archbishop Martin begged them to make space and finally he was able to embrace the weary priest.
"It's his day ... it's his day," said Archbishop Martin.
Once inside, Fr Mernagh thanked his supporters and all those who had joined him on the journey and together they recited the Magnificat, a prayer for the Virgin Mary.
The Archbishop thanked the priest for "his integrity and witness". Martin said little more but everyone knew the very act of receiving Fr Mernagh was a powerful message to Bishop John Magee.
Fr Mernagh, a humble priest, had called for him to step aside and undertook a personal walk of atonement.
Now, nine days later, the Archbishop of Dublin was endorsing the call in the most emphatic manner.
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Sotto Voce
(Source: II)