If you’re not able to fly to St. John's in January, then May is your next best option. Flight prices from Ireland to St. John's during May average around 394€.
In general, most travellers find the cheapest time to book a flight to St. John's is about 19 days prior to their departure date, when tickets on average are 551€.
The cheapest day to depart on your flight to St. John's is on a Tuesday when prices can be found for as low as 348€. This is often the case because there is less demand to fly during this day of the week. Flights from Ireland to St. John's on a Sunday will generally be more expensive and average around 723€. Keep in mind that these prices on both days could fluctuate depending on the season and the demand.