You may however have found a problem with the linking of John the Mate + Master to a Manx birth - I did a quick look on the UK National archives for John Taggarts and saw several from Ireland and one from Warrington in the army records - possibly I had the wrong selectors as I was looking for a possible Master's cert given the link with Dublin - the church St Marks then a Church of Ireland now somewhat different is near the Dublin docks - using the Channel Islands etc to avoid legal problems re marrying a deceased wife's sister is more common than often realised.
Is there any record of this Whitehaven John in census records?