Country code for Denmark is 45. Phone numbers are 8 digits.
Calling the UK from Denmark: 0044 + number
Calling Denmark from the UK: 0045 + number

Calling Ireland from Denmark: 00353 + number
Calling Denmark from Ireland: 0045 + number

Remember that there is a time difference of 1 hour! Eg. when the clock is 10am in the UK and in Ireland, it is 11am in Denmark.