The two matches come at the start of a crucial season for O'Brien's side, that culminates in the hosting of the 2019 UEFA European Under-17 Championships next May.
O'Brien's squad includes a number of the players from last year's winning Victory Shield side, and he is looking forward to working with the group.
"We've been working with this group since July, and had a training camp in August. These games will be full of opportunities for players and to see how they cope at international level."
With qualification already assured for next year's European Championships, O'Brien wants the team to face a high level of opposition as regularly as possible.
"Turkey will always bring a high level of opposition. We played them regularly over the past number of years. These games are preparation games, but there will always be that level of intensity to them."
Tallaght Stadium hosts the opening game on Tuesday evening. Kick-off is at 7pm.
Republic of Ireland Under-17 Squad
Gavin Bazunu (Shamrock Rovers), Darragh Burns (St. Patrick's Athletic), Conor Carty (Wolverhampton Wanderers), Thomas Considine (Bohemians), Kyle Conway (St. Patrick's Athletic), Jimmy Corcoran (Preston North End), Zak Delaney (West Bromwich Albion), Alex Dunne (Shamrock Rovers), Festy Ebosele (Derby County), Matt Everitt (Brighton and Hove Albion), Jack Funge (Bohemians), James Furlong (Shamrock Rovers), Leo Gaxha (Sheffield United), Joshua Giurgi (Norwich City), Brandon Holt (St. Patrick's Atlhetic), Cian Kelly (Shamrock Rovers), Seamas Keogh (Southampton), Timi Sobowale (Manchester City), Luke Turner (Aberdeen)
Fixtures
18/9/18 - Republic of Ireland v Turkey, Tallaght Stadium, KO 19:00
20/9/18 - Republic of Ireland v Turkey, Tallaght Stadium, KO 13:00