I am a little confused as to how any one would know that your MEIR was issued in Spain.
Well, they wouldn't, because it wasn't. Your IR was issued in the UK on the basis of training and testing carried out in Spain. Since any prospective employer will ask to see your logbook, and you are required to record in that logbook the airfields of departure and arrival, It is, on balance, likely that most interviewers would be able to draw the obvious conclusion. If, however, you were ashamed of having gained your IR in Spain, you could always falsify your logbook.