The opening clause does not have to be in the past for the mood tense to be. No es muy relevante is present, but if what follows refers back to the past, you use the past mood tense.
Era muy relevante que lo encontraran · Es increíble que vuelvan a hacerlo · Es increíble que volvieran a hacerlo
The trick of the track. Turn it is important that into the important thing is that, and the mood tense vanishes, because you end up with es que rather than an adjective plus que.
Use the lo plus adjective structure you already know: lo importante, lo increíble, lo espectacular.
Lo increíble es que vuelven a hacerlo
Irregular, but easy to hold: a line in the past is an era, and pronouncing that English word in Spanish gives you the form.
Era muy relevante · Era muy relevante que lo encontraran
Spanish shows the construction more plainly than English does. Creer is to believe, so creíble is believable and increíble is unbelievable. That is what incredible actually means.
Note there is no d in the Spanish.
Combining the two ideas of the track gives you a neat set of contrasts.
Lo increíble es que vuelven a hacerlo · Es increíble que vuelvan a hacerlo · Es increíble que volvieran a hacerlo
Same meaning, three positions on the mood-tense scale: none, present, past.
The reason for learning which structures trigger the mood tense is not only to use it correctly. It is also so you can steer around it when you want to.
Es importante que → lo importante es que