GOOD tightheads are worth their weight in gold and Wales hope they have struck it rich with Tomas Francis, even if the prop has spent his summer shedding a few kilos.

The Exeter front rower will make his long-awaited Test debut at the Aviva Stadium this afternoon, putting to bed once and for all questions over which country he will represent.

The Yorkshireman with a grandmother from Abercrave insists that decision was made long ago, much to the relief of Wales boss Warren Gatland.

With Samson Lee battling to make the second game of the World Cup against England following an Achilles injury, there is a heavy burden on the hefty shoulders of the uncapped Francis.

The 23-year-old has proved his scrummaging prowess with the Chiefs in the attritional Aviva Premiership and a strong display against the Six Nations champions in Dublin would not only cement his place in Gatland's final 31 for England 2015 but earn the 3 jersey in the opener against Uruguay.

"I know it will be a big step up, just like the Premiership is a big step up from the Championship," said Francis, who was playing for London Scottish little over a year ago.

"A tighthead is lucky in that if you do your basics, you can fit in to the cogs. I won't be making 50-metre breaks but if I can hold the scrum and give a platform hopefully I can make the step up.

"It still feel surreal and I have to pinch myself but it's here now and I have to try and take my opportunity."

"You are never going to have a perfect game and it's about resetting if you have a bad scrum. You can't back it up with another bad one," he continued.

"A tighthead's job is to anchor that scrum, be a part of the maul and make his tackles and rucks."

Gatland has been full of praise for the way that Francis has grafted over the summer, going from 138kg at the Six Nations to 130kg while reducing his body fat percentage.

The Exeter prop has the brains to go with the brawn yet he could only make the second team at the start of his mechanical engineering studies at Leeds University.

The student lifestyle led to him tipping the scales at a whopping 155kg but he is on the verge of the World Cup via Doncaster, London Scottish,Exeter and a gruelling summer.

"I feel in the shape of my life now – I am stronger and fitter," said the prop.

"In one evening session in Qatar we did a fitness test straight into a fitness session which was pretty severe.

"We were running same distance we had been doing in Wales but in the heat at the end of a long day. You couldn't cool down and your boots were squelching from the second rep.

"But nobody dropped out, everyone finished and pushed to the end. That's what it's all about and it makes you stronger and closer as a group. Those are the sessions that make you."

And at 2.31pm this afternoon, after training alongside Sam Warburton & Co last autumn and in the Six Nations, Francis will finally be an international rugby player.

"I'm very excited," he said. "Seeing some of the boys get capped a couple of weeks ago was a bit hard but to get my chance now, my goal is complete."

Ireland: R Kearney; D Kearney, L Fitzgerald, R Henshaw, K Earls; J Sexton, C Murray; J McGrath, R Strauss, N White; I Henderson, P O'Connell (captain); P O'Mahony, J Murphy, J Heaslip. Replacements: S Cronin, D Kilcoyne, T Furlong, D Ryan, S O'Brien, E Reddan, P Jackson, F Jones.

Wales: L Halfpenny, A Cuthbert, S Williams, J Roberts, G North, D Biggar, R Webb, G Jenkins, K Owens, T Francis, B Davies, A-W Jones (captain), D Lydiate, J Tipuric, T Faletau. Replacements: S Baldwin, P James, A Jarvis, L Charteris, J King, G Davies, R Priestland, H Amos.