* Ireland win first match in six to seal seeding
* Thrilling winger Gilroy leads rout, scores on debut
* Argentina must now rely on Australian win over Wales (Adds ranking update, quotes)
DUBLIN, Nov 24 (Reuters) - Ireland scored seven tries in a 46-24 demolition of Argentina on Saturday to ensure they will be seeded when the draw is made for the next rugby World Cup, and leave the South Americans waiting to find out if they will join them.
Ireland had to win the match, billed as the most important either side would likely play this side of the 2015 tournament, to book a place in the second tier of seeds for the Dec. 3 draw, and four tries in a blistering first half got them on their way.
Debutant wing Craig Gilroy got a first international try, as did the recently blooded Simon Zebo and Richardt Strauss, while Jonathan Sexton and Tommy Bowe got a brace each to maintain Ireland's unbeaten home record against Argentina.
"You never in your wildest dreams think you're going to get that number of tries against an Argentinean side but, if things click, they're well capable of doing that," coach Declan Kidney told a news conference.
Argentina, who only managed two late tries, fell out of the world's top eight and will stay there if Wales beat Australia next weekend. An Australian victory will push Argentina back in and see them avoid a potentially tougher World Cup pool draw.
Santiago Phelan's side began the day in sixth place but Samoa's narrow defeat to France was enough to provisionally push them ahead of Argentina in eighth by the narrowest of margins.
Argentina, who beat Six Nations champions Wales 26-12 two weeks ago to put them in the better position going into the game, simply had no answer to a rampant Irish side who put five successive defeats behind them in the best possible fashion.
Missing first-team regulars Brian O'Driscoll, Rob Kearney, Paul O'Connell, Stephen Ferris, Sean O'Brien and Rory Best, Kidney gave Ulster's Gilroy a first start and the 21-year-old flyer inspired the hosts to victory.
The winger, who scored a hat-trick in a non-cap win over Fiji last week, got his full international career off with a bang as he jinked past one Argentine tackler and between another two before touching down and being mobbed by team mates.
Sexton, whose inside pass had set Gilroy free, added the conversion and, while Nicolas Sanchez got back three points for the visitors, Ireland were back under the posts within minutes.
Gilroy, not yet an established starter at provincial level with Ulster, was involved again and more free-flowing rugby from the hosts culminated in Sexton exchanging passes with Leinster team mate Gordon D'Arcy before crashing over.
Sanchez was once more gifted a chance to trim the advantage but Ireland, who had managed only two tries in their previous five games, made it three in 20 minutes when Strauss sneaked into the corner for his first try in his second Irish start.
Argentina finally made it deep into Irish territory on the half-hour and were unlucky to come away with just three points but Ireland were at it again from the restart with Munster fullback Zebo joining the list of maiden try scorers.
Sexton missed his second conversion in a row but Sanchez was off target too with a penalty a few minutes later to leave Ireland a comfortable 24-9 in front at the break.
The flyhalves traded penalties in the opening minutes of the second half before the ever-commanding Sexton spotted a gap behind the Argentine back three for Ulster winger Bowe to speed through for his first try of the game.
Sexton's mixed day with the boot continued but he more than made up for it with a second try three minutes later as Ireland finally began to produce the kind of attacking rugby that has seen Leinster dominate the European club game.
The game, the 11th between the sides in 14 years, petered out in the last quarter with the main highlight being Argentina prop Maximiliano Bustos's sin-binning for throwing the ball in Cian Healy's face until Bowe got his 26th try for Ireland.
Replacement Tomas Leonardi and captain Juan Martin Fernandez Lobbe grabbed two consolation tries for Argentina whose minds now shift to Cardiff next Saturday.