The Hornets weren’t bad in the first half, Othmane Maamma and Luca Kjerrumgaard threatening, butt it was their total failure to respond to falling behind 10 minutes into the second half that defined the game.
Marko Stamenic tapped in a near-post flick-on from a corner to make it 1-0 before Josh Key scored a similar goal from open play to double the visitors’ lead 10 minutes from time.
Javi Gracia’s side were comprehensively outplayed in the second period, losing a game that rounds off a miserable run ever since their victory over Birmingham City on New Year’s Day.
And as the game drifted away, Irankunda saw red after falling for the bait from Liam Cullen and pushing him over. Tempers spilt over down the tunnel in a fittingly miserable ending.
After swapping ends to shoot towards the Rookery end first, Tom Ince had the first attempt of the game after 30 seconds, an effort rolling harmlessly wide.
Luca Kjerrumgaard heads wide (Image: Alan Cozzi/Watford FC)
But it was Swansea who had the better of the opening exchanges.
The visitors won a succession of corners, including a claim for a penalty when Stamenic was brought down in the box, before Zan Vipotnik saw an effort blocked from inside the box.
Marc Bola’s low cross was cleared by Lawrence Vigouroux, before the Swans keeper was called into action again to deny Othmane Maamma after a nice turn from Edo Kayembe.
Ince saw a tame effort saved before the winger’s cross found Luca Kjerrumgaard to head wide at the back pos.
Imran Louza and Kayembe saw long-distance efforts thwarted and, as Watford grew in dominance, Maamma picked out Ince at the back post, the winger sufficiently put off to not get any power on it.
As half-time approached, Vipotnik was picked out on the edge of the box but Selvik saved a weak effort easily, the two teams going in goalless.
Ince went very close early in the second half, cutting in on his left foot from a fast break but bending a shot agonisingly wide of the far post.
Tom Ince in action against Swansea City (Image: Alan Cozzi/Watford FC)
Moments later, one Swansea clearance released Ronald through on goal, but Bola did enough to get back and put him off inside the box.
At the other end, Kayembe dragged a shot wide as Watford made the running early in the second period.
But the Hornets were soon behind, in all-too familiar fashion.
A Swansea corner was flicked on by Vipotnik at the near post and tapped in by Stamenic as the Hornets conceded from a set-piece for the fourth game in five.
The visitors took charge of the game from there, Goncalo Franco forcing a save from Selvik as Watford failed to regain their previous momentum.
Othmane Maamma has an effort saved (Image: Alan Cozzi/Watford FC)
Kjerrumgaard powered an effort at goal but it was beaten away by Vigouroux, before Javi Gracia introduced Kwadwo Baah and Giorgi Chakvetadze off the bench to try and liven his side up.
Kjerrumgaard headed a Bola cross straight at the keeper but Swansea had taken charge completely, Gracia bringing on Irankunda for Mamadou Doumbia and replacing the under-par Imran Louza with Papy Mendy.
Soon enough, through, it was 2-0.
Stamenic ran through challenges from Chakvetadze and Kevin Keben, squaring for substitute Josh Key to tap in at the back post and essentially end the game as a contest.
Swansea continued to tease Watford, who had totally lost their way, and Irankunda’s late dismissal for a shove on Cullen was a sorry finale.
Watford: Selvik; Ngakia, Keben, Abankwah, Bola; Ince (Baah 64), Kayembe, Louza (P. Mendy 76), Maamma (Chakvetadze 64); Doumbia (Irankunda 76); Kjerrumgaard
Subs: Baxter, Mfuni, Eames, Grieves, Semedo
Attendance: 18,691 (1,455 away)