Conor Murray says Ireland must be "on their guard" when they face a Tonga team that will include "ridiculous athlete" Malakai Fekitoa in Saturday's World Cup Pool B match in Nantes.

Murray played alongside Fekitoa at Munster last season.

"He's a tough competitor, he really bought into our culture when he was at Munster," said the Ireland scrum-half.

Fekitoa, who won 24 caps for New Zealand before switching to Tonga under World Rugby's birthright amendment, is one of four former All Blacks - along with Charles Piutau, Augustine Pulu and Vaea Fifita - in Tonga's starting team.

The 31-year-old made 19 appearances for Munster and started in the Irish province's United Rugby Championship final win over the Stormers in Cape Town before leaving to join Benetton.

"He wasn't involved initially for the start of the season but he's an ultimate pro, he was always doing extras and looking after himself really well," added Murray.

"He fought his way back into the team and was a big part of our end-of-season run.

"He's such a nice guy and actually led really well throughout the year in terms of his rugby knowledge, obviously being an All Black and where he is now and he's been around the world.

"Really professional and a good leader. On the pitch he's a massive threat. He's really physical, a ridiculous athlete, and he's quite intelligent. He can bring in players like Piutau and set those guys free.

"Their backline is littered with guys of the size and pace of Malakai so we have to be on our guard for those guys."

While Ireland hammered Romania in their opening game under sunny Bordeaux skies, rain has been predicted for Nantes on Saturday.

The Irish were unconvincing when they laboured to a warm-up win over Samoa in wet conditions in Bayonne last month, and Murray admits they will be keeping a "keen eye" on the weather forecast before facing Tonga.

"You saw how that game went, it was a bit tricky for us," Murray said of the Samoa match.

"There was a lot of stuff, given the weather, that could've gone a lot better."

'Advantage Tonga if the game gets loose'

Ireland were given a scare by Samoa in their final outing before the World Cup with the Pacific Islanders building a 13-7 lead in the second half only for the Six Nations champions to claim a 17-13 win.

Munster's Murray was key to Ireland's victory, scoring a second-half try, but the 34-year-old admits Tonga will gain an advantage if Saturday's match unfolds in a similar manner.

"[Tonga are] a similar outfit," added Murray, who is playing in his fourth World Cup.

"We've obviously looked at them a lot this week, particularly in their powerful forward pack and their backline is quite explosive so similar to Samoa in that if the game gets loose, that's advantage to them a little bit.

"They do have structure around their free-flowing rugby as well. We've seen that over the week in the analysis. With these types of things, it can be tricky and get loose."

Ireland head coach Andy Farrell has named a strong team for the Stade de la Beaujoire contest, making just four changes from the side that started the 82-8 destruction of Romania as captain Johnny Sexton is retained at fly-half.

Murray and Mack Hansen replace Keith Earls and Jamison Gibson-Park in the backs, hooker Ronan Kelleher is in for Rob Herring at hooker and Joe McCarthy makes way for World Rugby player of the year Josh van der Flier.

Saturday's game is 15th-ranked Tonga's tournament opener having sat out last weekend.