Its not as straight forward as that. You'll probably find that many pilots use reduced thrust on takeoff from Shannon because of the longer runway. The reason they do this is to save fuel and reduce wear and tear on the engine, which increases it's lifespan.
Exactly many pilots do this for example, a few FR pilots at ORK i know (and correct me if i have quoted the wrong thrust quantity's) used to use just 19K thrust on the B738 for the DUB flights (very low loads) then on the ACE flights full 27K so runway length is not the issue pilots can demand the thrust setting they need and which is more economical, also to point out before it was axed CO used to operate the B752 full (and with more fuel then an ORK-EWR flight) and could take off safely with more thrust required, just like WW do at BHD to operate the B733 and B735 to European destinations.