Wikipedia:

Irish calendar

Common use Islamic · Gregorian · ISO · Astro · Julian
Calendar Types
Lunisolar · Solar · Lunar

Selected usage Armenian · Bahá'í · Bengali · Berber · Buddhist · Chinese · Coptic · Ethiopian · Germanic · Hebrew · Hindu · Indian · Iranian · Irish · Japanese · Javanese · Juche · Korean · Malayalam · Maya · Minguo · Nanakshahi · Nepali · Nepal Sambat · Tamil · Thai (LunarSolar) · Tibetan · Turkish · Vietnamese· Yoruba · Zoroastrian
Calendar Types
Original Julian · Runic

The Irish calendar does not observe the typical astronomical seasons (beginning, in the Northern Hemisphere, on the equinoxes and solstices), or the meteorological seasons (beginning on March 1, June 1, September 1 and December 1), but rather centres the seasons around the solstices and equinoxes (so that, for instance, midsummer falls on the summer solstice), beginning the seasons at the approximate halfway points between solstice and equinox, following the seasons of the ancient Celts (see below) which are pre-Christian in origin. This Celtic origin is particularly evident in the Irish naming of many of the months: some names, like May (Bealtaine), August (Lughnasadh/Lúnasa) and November (Samhain) were the names of pagan Celtic festivals. In addition, the names for September and October (Meán Fómhair and Deireadh Fómhair respectively) translate directly as "middle of autumn" and "end of autumn". Christianity has also left its mark on the Irish months: December is Nollaig, a word also meaning Christmastide.

Seasons

Winter - An Geimhreadh

November - Samhain / Mí na Samhna

December - Nollaig

January - Eanáir

Spring - An tEarrach (February, March and April)

February - Feabhra

March - Márta

The last days of March and the first three days of April are known as The Old Cows Days/The Days of the Brindled Cow or, in the Irish language, Laethanta na Riabhaí.

April - Aibreán