It's reassuring to know that for however long you might have been thinking about Ireland's Rugby World Cup squad, Joe Schmidt has been thinking about it for much, much longer.

Indeed it is safe to assume that Schmidt has been thinking about his squad since 19 October 2015.

As soon as Ireland once again fell short of a semi-final berth following defeat by Argentina, their head coach set the wheels in motion for the next World Cup cycle.

Over the past four years, everything Schmidt has done has been with a view to the 2019 World Cup.

His drive to get it just right this time stems from the idea that Ireland were caught short at the last tournament.

Injury and suspension meant that by the time Ireland arrived in Cardiff to meet Argentina, they did so without Paul O'Connell, Peter O'Mahony, Sean O'Brien, Johnny Sexton and Jared Payne.

Admittedly a particularly experienced core to be without, but the long and short of the matter was that Ireland's depth was not sufficient to cope.

And so, for four years Schmidt and the Irish Rugby Football Union have relentlessly sought to create strength in depth, to build a side that, as ruthless as it sounds, is capable of adopting a 'next man up' approach so that when the inevitable injuries arrive, the train keeps rolling.

Now, with a 43-man panel that must be trimmed to 31 by the time Ireland take off in September, Schmidt still has a number of decisions to make before finalising the squad that has been at the forefront of his mind for four long years.

1. The front row

With five props set to travel, Andrew Porter, Dave Kilcoyne and John Ryan will hope they have done enough to secure the replacement spots behind first choice props Cian Healy and Tadhg Furlong.

The wildcard comes in the form of Jack McGrath, a British and Irish Lion in 2017, who recently moved north to Ulster having fallen down the pecking order at Leinster. The prop, with 54 caps to his name, will have to make a serious impression if he is to force his way into the 31.

Like McGrath, Connacht's Finlay Bealham will hope he is given a chance to put forward his case in the coming weeks.

At hooker, Sean Cronin's long-awaited first international start was a disappointing one, but his stellar season with Leinster leaves him well placed to continue as chief support to skipper Rory Best.

Ulster's Rob Herring must impress against Italy if he is to move ahead of Niall Scannell in the pecking order.

2. The back row

Twelve months out from the World Cup it seemed as though openside flanker would be one of Ireland's strongest positions with O'Brien, Dan Leavy and Josh van der Flier all staking a claim for the starting jersey.

However injuries to Leavy and O'Brien leave Van der Flier as the obvious shout at seven. Beside him is likely to be CJ Stander, although the ever-impressive Jack Conan will offer genuine competition at the back of the scrum. Peter O'Mahony is a sure thing at blindside.

If just one more back row place is available, it will be contested by Tommy O'Donnell, Jordi Murphy and Rhys Ruddock, all of whom will start on Saturday.

Both Murphy and Ruddock offer considerable international experience, with the former's move to Ulster this season in search of more game time keeping him firmly in the mix for Ireland selection.

Ruddock, a trusted lieutenant of Schmidt's during his time as Leinster head coach, will captain the side on Saturday while O'Donnell will mark an impressive return to the international stage having played just three times for Ireland since dislocating his hip in 2015.

Another point of consideration is that Iain Henderson and Tadhg Beirne, both of whom will probably travel as locks, have experience playing in the back row and could slot in should Schmidt deem it necessary.

3. The half-backs

As evidenced by Joey Carbery's departure from Leinster and the non-renewal of Ruan Pienaar's Ulster contract, the IRFU have not been afraid to force the hand of the provinces to ensure that Irish players are receiving adequate game time.

In both instances, the decisions have undeniably benefitted Ireland.

Carbery has thrived as the established first choice fly-half at Munster, while Pienaar's departure paved the way for the emergence of John Cooney, who has since twice been named in the Pro14 Team of the Year.

At scrum-half, Schmidt has a strong supporting cast behind Conor Murray while Carbery is sure to travel as Sexton's understudy.

With at least five half-backs set to make the cut it's probable that there will be places for two of Cooney, Luke McGrath and Kieran Marmion.

In recent years Connacht's Marmion has been Schmidt's preferred option when Murray is not available, while Cooney's place-kicking may just give him the edge for the third spot, although McGrath will have a chance to impress from the start against Italy on Saturday.

Another curveball comes in the shape of fly-half Jack Carty, whose excellent form at Connacht last season might force Schmidt into considering bringing Carty as cover for 10 and 15 at the expense of another back.

It certainly appears as though Carty sits ahead of Leinster's Ross Byrne, who is not in the squad for Ireland's first warm-up game, in the current pecking order.

4. Utility backs

Schmidt already has a headache trying to decide who out of Robbie Henshaw, Bundee Aki and Garry Ringrose to leave out of his starting line-up for Ireland's Pool A opener with Scotland on 22 September.

Munster's Chris Farrell has impressed in all five Ireland appearances, proving himself to be rock solid in the defensive line while carrying a real threat with ball in hand. Although not a certainty, he is well placed to make the final 31.

Jacob Stockdale, Keith Earls and Rob Kearney are all clear starters with Jordan Larmour providing the chief support on the wing and at full-back.

Assuming Schmidt chooses to bring 14 backs as he did in 2015, that leaves just one seat on the plane remaining.

Dave Kearney and the uncapped Mike Haley will both have a chance to shine against Italy on Saturday but as it stands both would be considered outside bets.

It looks as though Andrew Conway and Will Addison will battle it out for the final spot.

Both are capable of slotting in anywhere across the backline, with Conway notching a hat-trick against the USA when given the chance to start last November.

Addison, who Schmidt persuaded to join Ulster from Sale last summer, impressed early in his time in Belfast before injury cut his season short, but his late call-up to the World Cup panel shows Schmidt is willing to give Addison the chance to prove his fitness.