For the same reason that some Americans are dark skin...
Some Irish people do, but many don't. It is the genes of some Irish people in the same way various traits can be found in other countries.
It's crap
bioluminescence. Chemicals in their skin.
Most probably because of the 1916 rising and events leading up to this, whereby the Irish rebels fought against the British rule and brought independence to Ireland.
some boys do in some boys don't
No, they are fair skinned and red headed. Dont listen to the above. Yes some Irish people are fair skinned and light haired, but some are also dark haired and have a more swarthy skin complexion. These people are believed to be the descendants of the original Celtic tribes that came to Ireland and Britain from southern Europe. This darker complexion is also quite common is Wales and South west England. The fair skinned light haired Irish are most likely of Nordic descent.
Tráthnóna (before dark); oíche (after dark)
Kieron is an Irish name. It means 'black' or 'dark'.
Be Irish
The name Kieran is of Irish origin and it means dark. With an Irish origin Kieran means "little dark one".
Irish: craiceann Scots Gaelic: craiceann, sgrath Welsh: croen