Are you aware of the purpose of Griffith's Valuation? When it was published, and how long it took from Day 1 to completion? Are you aware of why a person's name was recorded by Griffith?
The Griffith's Valuation was NOT a list of all inhabitants, NOT a list of all voters, not a list like modern telephone directory. Given that there was (just) one John Purtell named in Griffith's Valuation does NOT mean there were no other John Purtell (or the alternative spellings, such as Purtile).
In order to be sure this one John Purtell sighted in Griffith's Valuation, is your ancestor, secondary evidence is required. How do you know there were not 3 other men named John Purtell residing nearby at the same time who were labourers?
In relation to Honorah, there is no interest or value in revealing her private life, which you should respect. However, from my reading of your response it appears the scenario you are building is - in simple words - young girl migrates with parents to USA, is abandoned, returns alone to Ireland, to grandma, then marries a neighbour - all this before her 15th birthday.
It's great to have an hypothesis and then find evidence to support the theory. This is standard reseach methodolgy in all the sciences, that also applies to genealogy. Sound investigation methods, rather than having an ASSumption (sic), will yield plausible results. How did young abandonded Irish girl return from USA to Ireland? Have you found her name on a passenger list? Who provided funds for her return? Did she sail alone? Was she sufficiently literate to know how to get back to Ireland? Which port did she disembark? How did she make her way back to grandma's house? To support your notion (ASSumption), facts and/or plausible information are vital to good research.
The age of 14 years at marriage is the youngest I have ever seen.
If Purtell is the one mentioned in Griffiths, for a particular townland location, then it is most unusual that the family would be associated with 4 different townlands in Kilcoman parish. Normally, the family stays put in the same townland, especially if they were lessees as recorded in Griffith's Valuation. Successive townland residences recorded with baptisms indicate transient labourers or journeymen.
You asked: I have no idea which church or chapel they married in. Why does it matter? This is a good question. Presuming you've found a corresonding record in the NLI films, these can be used to finetune the location of your ancestors. With the NLI films is information as to the source (within RC Dicoese) of your selected record. This location allied with the date can be used to pinoint the Chapel or Church where the ceremony took place. For many of Irish descent, being able to visit that place and stand where our ancestors stood is important. For you, it might not be.
There is an excellent free website swilson.info which provides all sorts of treasure - too many to list here. Specifically, dates and locations of both Church and Chapel, linked to modern maps, are great for planning a tour of your ancesctors' homelands. In one case I dealth with, the County of Marriage was different to the known county of residence. Hence there was uncertainty - right family or wrong family? The vital question.
By reference to <swilson.info> the reason emerged. The small Chapel where the folk lived in a remote area, had been closed by the Church just 1 year before the firstborn child, As a result, the congregation had to walk a difficult terrain, cross a river and arrive at the closest RC place of worship for marriage or baptism. In doing so, they were actually in a neighbouring county. As a result the people I was helping could now continue with confidence.
As a matter of accuracy, 'everything is important in Irish family history'.