Thanks for explaining all that. I assumed he'd joined the army as a lad, either in Ireland or England, and served until the end of the war with France and beyond. I recall reading something about the regiment not being sent to Waterloo (1815) because it was so under-strength through death, injury and disease after previous campaign. (I may have mixed it up with another regiment; there would have been several in a similar position after the Peninsular War.)
He may have learned weaving as a youngster in Ireland. It would have been flax/linen-weaving. Linen manufacture was one of Ireland's few successful industries at the time. Ulster is well-known for it. (My 1st choice of linen is always Irish.) It was also produced in other Irish regions. My GGF who lived in Mayo throughout most of the 19thC (he had a long life) was a small tenant-farmer. He had another source of income from a part-time, seasonal job as a flax inspector. He used to travel around Mayo and neighbouring counties in Connaught. I don't know any details of his work. He would have needed to be literate and numerate to keep records. He was born around 1810, so the next generation after John Purcell. He would have been bi-lingual, as were his wife and children. His sons and younger daughter, born in the middle third of the 19th century could read and write in English, elder daughter could only read, his wife could do neither. In the 1790s an effort was made to encourage the Irish linen industry through a grant scheme. Anyone growing more than a certain acreage of flax was eligible to apply for a free loom. The qualifying acreage wasn't large, only a few acres. Some lists of recipients survive.
I wouldn't have thought literacy level was high in Ireland at the end of 18thC. but have no documentary evidence for that opinion. There were "hedge schools", unofficial schools, sometimes held outdoors. Penal laws against Catholics forbade them being schoolmasters, so "hedge schools" were illegal. Enforcement of these laws was lax by later in 18th century and from 1770s onwards a series of Catholic Relief Acts abolished or eased many of them, culminating in the Catholic Emancipation Act at end of 1820s. If your man was Protestant he stood a better chance of an education. Also if he was Protestant his family may have been better-off financially, since the Penal laws restricted what Catholics could own, what they could leave to their heirs, and what occupations they could follow.
You say you haven't found any christenings of children of the 1st marriage. If there were children and if John was R.C. they may have been baptised as R.C.. I doubt if there would have been many Catholic priests in Norfolk at the time, so any potential baptisms may have happened years after birth and not formerly recorded. But if they did exist, what happened to the children? Marrying by licence is sometimes also a clue that one party to the marriage may not have been of the Established Church. There were other reasons for marrying by licence as you suggest. Another thing to consider is that a marriage licence cost money, so either the bridegroom or his family or someone had a bit of spare cash. Would a working man still in his teens have enough cash? Was there a bond with the licence? An alternative reason for a licence was if the wedding had to take place at short notice, e.g. if groom was going away on military service or if bride was in an advanced state of pregnancy. In some cases if the young woman was "big with child" and poor and the baby's father was also poor and seemed reluctant to do his duty, the parish might pay for a licence, because it was cheaper than providing poor relief for a single mother & child and then trying to chase an errant father for costs. Perhaps in this case the bride's family paid for the licence. As she was a mature bride they might have been glad she'd found someone to marry her and wanted a quick wedding before the young bridegroom had 2nd thoughts.
Take a look at RootsChat Irish sub-board. There are several sources for Irish research now, some of them free. You can look at images of parish registers for free. Irish National Archives has online tutorials. There are other useful introductions to Irish research free online. You need to know what records existed, which survive and which were lost or destroyed. 2 of the biggest fee-paying FH sites usually have a free week-end for St. Pat's Day. I took part in an Irish research online tutorial with Q&A on one this year.
At least you know where in Ireland to start your search, which is an advantage many don't have. How did you identify his place of origin?