This website has information about the route including details of a couple following the way by train
http://friendsofcolumbanusbangor.co.uk/from-bangor-to-bobbio-saint-columbanus/
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Bangor Co.Down is 10 miles from, where I live,Belfast.There is a beautiful coastal walk from a small town called Holywood to Bangor.
Columbanus left from Smelt Mill Bay,on .the coastal path,which leads to Bangor .
There is an information board on this spot depicting the monks leaving in their rowing boat with a Map detailing their journey.One of the places they landed at was Bregenz in Austria which is today twinned with Bangor.
Columbanus lived in Bangor Abbey and there are guided tours of the Abbey in the tourist season.
The coastal path is beautiful ,especially in the Spring when there are lots of bluebells.Weather permitting you can enjoy a swim in Helen's Bay ,a pretty village on the coast.
The train from Belfast to Bangor follows the coastline with access to the coastal path at several stops along the way.
Hope you have the opportunity to walk this path it's not to be missed.