Long, 27, has signed on a four-year deal for a fee that could reportedly rise to around �12 million ($20 million, 15 million euros), according to British media reports.

Gardos, 25, has also signed a four-year contract and is reported to have cost Southampton around �6 million.

On Long, Southampton manager Ronald Koeman told the club's website: "Shane is a very good player. As a striker, he's different to (fellow new signing) Graziano Pelle, which is something we needed.

"He brings ambition and a great mentality. He's always running and always working, and he's very down-to-earth. He's a great person, and we need that type of player."

Southampton finished eighth in the Premier League last season, but have since lost manager Mauricio Pochettino as well as first-team quintet Adam Lallana, Luke Shaw, Rickie Lambert, Dejan Lovren and Calum Chambers.

But as well as Long and Gardos, they have also signed Pelle, Dusan Tadic and Fraser Forster, and loanees Ryan Bertrand and Saphir Taider.

"I'm delighted," said Long, who only joined Hull from West Bromwich Albion in January.

"This came out of the blue for me, but once I'd come and had a look at the training facilities and met the manager, it seemed like the right decision for me and my family.

"It was a hard decision for me to make, but when you come down to Southampton, you see how big a club it is and how well set up it is."

Long joined Reading from Irish side Cork City in 2005 and spent six seasons at the Madejski Stadium before signing for West Brom in 2011. He has made 47 international appearances for Ireland and scored 11 goals.

Gardos has won 12 caps for Romania and has been signed to replace Croatia international Lovren, who followed Lallana and Lambert to Liverpool.

"From the beginning, after Lovren left the club, we paid a lot of attention to Florin," said Koeman.

"Everybody knows the big names and the big players, but it's always more difficult to find one who's not so well known. The information the club has given to me about him has been very good."

Gardos said: "This is one of the happiest days of my life. It's the biggest step of my career so far.

"I've had a very good first impression of the club. There's a very big difference between what I had back home and what is here."

Southampton open their Premier League campaign away to Liverpool on Sunday.