This song still puzzles me, so here's a bit more, though not much:

I've listened yesterday night to Nic Jones singing Isle of France, to the Johnstons singing Newry highwayman and to Martin Carthy singing Newlyn town. Jones' tune is much slower so it's hard for me to compare, but I do not hear more than a superficial similarity. What Nic Jones sings, however, has not too much in common with the two tunes given for the Isle of France in Karpeles (ed.), Cecil Sharp's collection Vol 2. (But I'm not particularly good in reading the tune from print, so it still might be a variant).
In the longest version I have read, the coastguard actually saves the convict from the sea and the convict did come from, not to the Isle of France after having served there 6 of his 7 years. His ship, the 'Shamrock Green' had been shipwrecked.

Martin, "a ring and chain" it is in the three printed versions I have, as you have presumed. The second line of verse 3 I have as "we were coming home for to make up one(?)" which doesn't make it clearer, at least for me.

Here's part of the notes to this song from Ewan MacColl's 'Traveller's songs from England and Scotland':
MacColl first retells the whole story (for the version he prints is too short to understand the story), wrongly places the Isle of France as one of the Channel Islands and then goes on: "The song has been collected chiefly in England and the references all give a melody similar to ours. Kidson has a 'strong suspicion that the balld was founded on a real escape from a convict transport ship'.
Bibliography:
British: JFSS, Vol 1., p.123; Vol 2, pp. 258-9; Sharp (1), pp. 232-3; Sharp and Karpeles (2); vol II, pp. 143-5....[several broadsides; I could type them if it helps for the questions]...
Australian: Edwards, pp. 5-6.
Alternative title: The Shamrock Green, The Convict Song"

Wolfgang