Ryanair condemned the strike announced by Forsa, a powerful trade union in the Irish public service sector, which represents the interests of Ryanair's Ireland-base pilots.
Norwegian is seeking to hire captains and first officers certified to fly Boeing 737 aircraft and will nearly double the number of flight crews at its Dublin base, it said on Friday.
Warning investors on Monday of more strikes this summer, Michael O'Leary said he would consider moving aircraft from Ireland, and and acted swiftly within 48 hours.
Ryanair's directly-employed pilots in Ireland will join colleagues in Sweden and Belgium on strike in an escalating day of action on August 10, the fifth one-day walkout in the airline's home market.
Unrest surfaced at Ryanair following a planning mix-up in September 2017 which led to 20,000 flights being canceled.
Kenny Jacobs, chief marketing officer at Ryanair, commented: "Regrettably, nearly 200,000 Ryanair customers had their flights cancelled in July because of repeated ATC staff shortages in the UK, Germany and France, adverse weather, and unnecessary pilot and cabin crew strikes".
Ryanair's pilots based in Belgium and Sweden have announced that they will strike on August 10, a date that coincides with the strike date just announced by Forsa.
The airline is bracing itself for another round of strike action next Friday, when 25 per cent of its pilots will strike.
The Svensk Pilotförening (SFP) union said it had tried to start negotiations with Ryanair over the past eight months without success, and that at an April meeting the airline refused to accept the union delegation and wanted to choose the union's negotiation team.
The airline said the latest action had no effect since all 3,500 customers (of the 430,000 travelling with Ryanair on Friday) were re-accommodated or refunded last weekend.
Ryanair has said it will shift jobs and planes to more profitable areas, and threatened to move more if the strikes continue.
But employees have long slammed their working conditions.