In my school hymn book I always see numbers and a name at the start of a hymn, eg
Slane 10 11 10 10 Or 'irregular'
What do these mean? Are they indications for the organist?
They represent the number of syllables that fit to the lines of the tune. Thus, any hymns with the same series of numbers can be sung to any other (with the same numbers). And vice versa.