Barker will partner the newly-crowned Ulster Rugby Personality of the Year Johann Muller in the second row. Neil McComb has been added to the replacements bench.
For their second knockout match of the season, Ulster will be without the services of flanker Willie Faloon who injured his shoulder in training two weeks ago. He will undergo surgery today to repair a torn rotator cuff.
Pedrie Wannenburg, Chris Henry and Robbie Diack are retained in the back row and it promises to be a physical battle at the breakdown against Leinster's much-lauded loose forwards.
Ruan Pienaar, who was crowned Magners League Player of the Year last Sunday, and Ian Humphreys continue as the province's starting half-backs.
Previewing the semi-final, loosehead prop Tom Court said: "We are going down to Dublin under no illusions. Leinster are playing fantastic rugby, but if you get on top of them early I think they'll realise they're not as good as everyone tells them they are.
"They are definitely beatable and we saw that (last month) against Munster in Limerick. To have any sort of chance against Leinster you need to combat them physically for those first 20 to 30 minutes.
"You need to be starting well with huge physicality, match them up front and make them realise you're here for a game and that it's not just going to be a Sevens match.
"We gave it a good rattle for 60 minutes (against Northampton), so obviously for us it's now about getting in the same place but making sure we hold out for the last 20 minutes.
"Leinster are a very good rugby team with great players but it's a one-off game and on the day I think if Ulster get in the right place and get the physicality up, then we're definitely in with a chance."
ULSTER (v Leinster): Adam D'Arcy; Craig Gilroy, Darren Cave, Nevin Spence, Simon Danielli; Ian Humphreys, Ruan Pienaar: Tom Court, Rory Best (capt), Declan Fitzpatrick, Johann Muller, Tim Barker, Pedrie Wannenburg, Chris Henry, Robbie Diack.
Replacements: Andi Kyriacou, Paddy McAllister, Jerry Cronin, Neil McComb, Thomas Anderson, Paul Marshall, Ian Whitten, Conor Gaston.