why Noregian airfare from Stewart to Shannon is so high ($853) for Oct.?? Paid $265 last Sept....

They may have seen Wow Airlines last month and a couple of other low cost transatlantic airlines go out of business.
Norwegian Air was in financial trouble at the end of 2018.

LOL...I had the same thoughts.

Or perhaps they just made a bad bet on the price of jetfuel.

It's interesting that a round-trip ticket to Europe costing $853 is considered "so high". One could just as easily wonder how a round-trip ticket to Europe priced at $265 would allow any airline to stay in business for very long.

Not much about ticket pricing makes much sense to most humans.

Maybe that $265 was one way?

It's like asking about stock price fluctuations. If you don't like what you see, just wait and look again. Fares go up and down and the seat inventory is subdivided into different tranches, all with their own price ranges. It's only October. There are too many variables involved in dynamic pricing algorithms for anyone to tell you the exact reason(s) .

It all about dynamic free market pricing to maximize an airlines profit. With Norwegian targeting the leisure market where one option is not to go, price swings are more dramatic than airlines depending more on the business market where travel is less of an option. And the existance or disappearance of other low cost competition can also drive pricing.

I noticed that too. We flew RT Providence to Dublin for $333 last June. Checked to see if maybe we could go again this summer or fall and it's over $1600 from July on now.

Why?

Because $265 airfares to Europe are unsustainable. They lead to airlines that abruptly go belly up like Primera Air and WOW Air. Struggling Norwegian is trying to avoid going down the same road by righting previous ill-advised decisions, like placing cabin crew and pilot bases at Stewart and flying 27 flights per week to Europe from a secondary airport like Stewart, which required rock-bottom fares to fill planes. Further headwinds for Norwegian have included engine problems on its Dreamliners and more recently the grounding of its 18-plane 737 Max fleet.

https://www.recordonline.com/news/20190117/questions-about-norwegians-future-at-stewart-airport

https://skift.com/2019/03/15/norwegian-airs-problems-go-beyond-boeing-737-max/

Many airlines were affected by the 737 Max grounding, NA was not unique.

Here’s a positive article from Forbes Magazine on the financial state of NA:

https://www.google.com/amp/s/www.forbes.com/sites/grantmartin/2018/12/24/norwegian-air-shares-optimistic-forecast-amidst-financial-scrutiny/amp/

We’re flying NA for the 2nd time in Sept. Tickets for SF (Oakland) to Paris, r/t, Non-Stop, are about $450 right now. Even if you add the $90 each way for reserved seat, checked bag and 2 meals, it’s a bargain. I love NA and especially the Dreamliner.

Because $265 was way too low a price for that trip. That fare was a money loser for the airline and so could not last forever. It was either increase the fares to something that can sustain the business or stop flying like WOW or cancel flights like Jet Airways (India) recently did. You got a deal last year. The single biggest predictor of low fares on a route is the existence of a competing budget carrier on the route. Is there one for this route? If you can fly from New York City where there are multiple airlines flying to Ireland, you might find lower prices but more in the $550 to $650 range for a nonstop flight.