Mono bouquets in Dublin: one flower, chosen by the Irish season
Ireland grows only a small share of the cut flowers it buys, but its growers still work to a season: narcissus in spring, dahlias through summer and into autumn, sunflowers until mid-autumn. Each of those three has its own section among the mono bouquets in Dublin. Choosing one variety usually comes down to the month and to what the recipient wants standing in the vase.
Spring narcissus, late-summer dahlia: flowers Irish growers cut too
Flower Farmers of Ireland counts more than a hundred members, among them growers in County Dublin. Between them they cut narcissus in spring, dahlias from summer into autumn, and sunflowers until mid-autumn.
Narcissus, dahlias and sunflowers each have their own section among the mono bouquets, but buying from one of those sections doesn't mean the stems grew in Ireland. The Irish season only tells you the flower is in its month here; most of what reaches Dublin's florists still arrives through the Netherlands.
Narcissus is worth keeping to its own bunch for a practical reason: its cut stem leaks a sap that shortens the life of other flowers sharing the same water. A bunch bought for a mixed vase at home should stand alone in its own water for about a day first, and the stems shouldn't be given a fresh cut afterwards.

Tulips or lilies: the two flowers Dublin can buy in any month
The family-run stalls on Grafton Street keep roses, lilies and tulips on sale all year, and bring lilies back for Christmas. That makes tulips and lilies the two single-variety flowers on offer whatever the month.
Between the two, tulips suit an everyday gesture, something sent without much occasion behind it. Lilies read as the more deliberate choice for a winter visit, since the same stalls keep bringing them back through the Christmas weeks.
One household where lilies are the wrong choice is any home with a cat: every part of the plant, and even the water left standing in the vase, is toxic to cats. Tulips make the safer alternative there.
Hydrangea in Dublin: a summer stall flower, a few heads to a bunch
Grafton Street's summer stalls sell peonies alongside hydrangea, and hydrangea has its own section among Dublin's mono bouquets; one of the names there is Bunch of Pink Hydrangea.
Each hydrangea flower head is large enough on its own that only a few stems make a full bunch, fewer than most other single-variety flowers would need.
Hydrangea also drinks more than most cut flowers and takes up water through its petals as well as its stem. A head that has drooped can be revived by lowering it into cool water for an hour or two; leave it no longer than four or five hours.

The florist photographs each bouquet before the courier sets out, delivery is free across the centre, and most orders reach their recipient the same day they're placed; the exact window shows on the bouquet's own page, next to mono bouquet delivery in Dublin. That leaves a short route through the mono bouquets in Dublin: narcissus in spring, sunflowers running until mid-autumn, and tulips or lilies whichever month you're buying in.