I think the OP meant paying initially at the repayment level for a 30 year term albeit on a notional 40 year term, but then having the flexibility to reduce the payments to the lower level required to repay in the 40 year term if having financial difficulty.
To the OP: I don't see any major obstacle* or downside, provided that you are on a tracker rate with the greater flexibility that allows for overpayment. You'd need to check the terms and conditions of any loan offer. And as ClubMan points out, you will have higher life assurance premiums.
*Note however that 40 year terms are still relatively unusual in the Irish market, and many lenders will shy away if the term would exceed the balance of your working life. So if you're now 25 and expect to retire at 65, there may not be any problem; if, however, you're 37... I wouldn't expect lenders to jump at the chance of lending to you!