Kept them up late at night in Irish leader's mansion, composing these letters (8)
I believe the answer is:
insomnia
'kept them up late at night' is the definition.
I can't tell whether this definition defines the answer.
'irish leader's mansion composing these letters' is the wordplay.
'leader' suggests taking the first letters.
'composing' means one lot of letters go next to another.
'these letters' indicates anagramming the letters.
The first letter of 'irish' is 'i'.
'i'+'mansion'='imansion'
'imansion' is an anagram of 'INSOMNIA'.
'in' is the link.
Can you help me to learn more?
(Other definitions for insomnia that I've seen before include "problem at night" , "Inability to drop off" , "Condition affecting sleep" , "Sleep disorder" , "State of chronic sleeplessness" .)