Ryan O’Shaughnessy secured their place after singing his ballad Together in the first semi-final in Lisbon.
The UK, as one of the “Big Five” countries, automatically qualified for the grand final on Saturday.
O’Shaughnessy’s ballad was accompanied by two male dancers acting out the romantic story of a couple, which got a huge cheer from the audience.
With seven victories, Ireland is the most successful country at the contest, but finished in last place at the final in 2013 and hasn’t competed since.
If there was a prize for most eccentric performance, it would have to go to Israel – whose singer clucked like a chicken on stage.
Netta Barzilai, who performed her song Toy, has been the favourite with bookmakers to win this year’s competition.
She has been dubbed “Bjork on steroids” for her energetic performance.
The first of the two semi-finals – with the second to air on Thursday – also saw former X Factor contestant Saara Aalto make it through to Saturday night, representing Finland.
The UK will be hoping for its first Eurovision win in more than two decades.
Singer-songwriter SuRie is representing the UK with the song Storm.
Here are the first 10 countries to qualify: Ireland, Austria, Estonia, Cyprus, Lithuania, Israel, Czech Republic, Bulgaria, Albania, Finland.
Performers from Greece, Switzerland, Armenia, Croatia, Macedonia, Belarus, Belgium, Iceland and Azerbaijan went home.
Portugal is hosting the annual event this year after its contestant Salvador Sobral won last year’s show in Ukraine.
The Eurovision Song Contest final will feature 26 contestants and airs on Saturday at 8pm on BBC One.