The current rule
Ireland moves forward at 1:00 a.m. UTC on the last Sunday in March and back at 1:00 a.m. UTC on the last Sunday in October, in step with the United Kingdom and every EU member state. Winter time is UTC+00:00 and summer time is UTC+01:00.
A genuinely unusual legal definition
Ireland is the rare country whose law defines summer as standard time rather than the other way around. Under the Standard Time (Amendment) Act 1971, Irish Standard Time is UTC+01:00 and applies in summer, with the country reverting to GMT in winter as a form of negative daylight saving. The clocks land in exactly the same place as everyone else's, but the legal framing is inverted — and some timezone software correctly reports Ireland as being on standard time in July and on a winter deviation in January.
The border question
Because Ireland and the United Kingdom change on the same dates, there is never a period where clocks differ across the Irish border. That alignment would end if the EU abolished seasonal changes and the UK did not, or vice versa, which is one reason the proposal has proved politically awkward.