From April to July every year, bee colonies will naturally split. Half of the bees will leave their nest to find a new home. This is caused by many factors and is a natural occurrence within a colony. As the bees travel to a new home they will move in large numbers which is quite scary for some people, however, they mean no harm. The name for it is swarming.
Swarms can often be problematic in urban environments as the bees can move into man-made structures - chimneys, roofs, walls etc. The bees can be difficult to remove and are often exterminated. At OpenHive we launched a program last year to help with this. You can read about it here.
If you live in south Dublin, north Wicklow or Galway and need help with a swarm, feel free to get in touch.
A wonderful resource for help with swarms in Ireland can be found here: https://swarms.ie/