The Irishman was suspended for three matches following a stamping incident against London Wasps on February 15.
But he has been recalled to the starting line-up in place of Darren Allinson for the trip to Quins.
He partners Shane Geraghty at half-back and Jimmy Stevens starts in the front row in place of David Paice.
Matt Parr, Jimmy Stevens and Jamie Hagan make up the front row with George Skivington and Nic Rouse in the second row.
Kieran Low and Blair Cowan will play alongside Ofisa Treviranus in the back row.
Eamonn Sheridan and Fergus Mulchrone are the centres while Marland Yarde, Topsy Ojo and James O’Connor make up the back three.
Irish director of rugby Brian Smith said: “I’d like to thank our supporters for giving Declan Danaher a wonderful send off last Saturday and encourage them to support the team this weekend at the Stoop.
"Harlequins are a very good team and we are looking forward to the contest tomorrow.”
London Irish team to face Harlequins: 15. James O’Connor; 14. Topsy Ojo; 13. Fergus Mulchrone; 12. Eamonn Sheridan; 11. Marland Yarde; 10. Shane Geraghty; 9. Tomas O’Leary; 1. Matt Parr; 2. Jimmy Stevens; 3. Jamie Hagan; 4. George Skivington (Captain); 5. Nic Rouse; 6. Kieran Low; 7. Blair Cowan; 8. Ofisa Treviranus.
Replacements: 16. Mike Mayhew; 17. John Yapp; 18. Leo Halavatau; 19. Jebb Sinclair; 20. Gerard Ellis; 21. Alex Lewington; 22. Myles Dorrian; 23. Darren Allinson.
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