The island of Ireland functions on Greenwich Mean Time (GMT)
GMT is an absolute time reference and doesn't change with the seasons.
The time on the island of Ireland is the same as Greenwich Mean Time less than half of the year.
During the summer it becomes GMT+1, that's Irish Standard Time (IST) in the Republic and British Summer Time (BST) in Northern Ireland.
BST and IST start from 01:00 GMT Sunday 31 March 2013 until 01:00 GMT Sunday 27 October 2013.