My preferences

I like drinks that are sweet, fruity, smooth, and easy to drink — not too intense, not carbonated necessarily. Specific favourites:

Sweet plum tea (maesil-cha, 매실차) — had in Korea, loved it
Tesco Plum & Pear Still Flavoured Water — UK only, not stocked in Tesco Ireland
Pocari Sweat / Gatorade — mild sweet electrolyte drinks, had in Korea

I cannot eat chocolate or caffeine, so hot chocolate, coffee, and regular tea are all out. I live in Ireland, so availability matters.

Korean plum tea (maesil-cha)

This is the closest match to what I loved. Two ways to get it in Ireland:

Buy maesil-cheong (plum syrup) from an Asian store:

Asia Market — Ireland's largest Asian supermarket (Drury Street or Ballymount, Dublin). Stocks Korean syrups and drinks.
Hanyang (한양) — Korean grocery in Dublin
Look for bottled maesil-cheong (매실청) or maesil-aek (매실액) — brands like Ottogi, CJ, or Hong Ssang Ri
To serve: 2-3 tablespoons syrup + water (hot, cold, or sparkling). That's it.

Make it from scratch (summer project):

Equal parts green plums + sugar in a jar. Leave in a cool dark place for 3+ months. Strain.
Green plums (or regular unripe plums) may be available at Asian stores or farmers' markets in June-July.
The syrup lasts ages and gets better with time.

Recreating plum & pear flavoured water

Since Tesco Ireland doesn't stock the UK plum & pear water, DIY is easy:

Maesil syrup + water — already close to what you want
Pear juice + plum juice diluted — mix fruit juices with still water to taste
Fruit infusion: slice ripe pears and plums into a jug of water, refrigerate overnight
Squash option: Robinsons or MiWadi fruit squash diluted very weak gives a similar light-fruity-water vibe

Pocari Sweat / Gatorade alternatives in Ireland

The mild-sweet-electrolyte category. What's actually available:

Lucozade Sport — widely available in every Irish supermarket and Centra/Spar. The closest mainstream equivalent to Gatorade.
Powerade — available in Tesco Ireland, Dunnes, SuperValu
O.R.S. Hydration Tablets — dissolve in water, available in pharmacies. Less sweet but effective.
SiS Go Electrolyte or High5 Zero — available in sports shops and online (e.g., Life Style Sports, Elverys)
DIY electrolyte drink: 1L water + 1/4 tsp salt + 2-3 tbsp honey or sugar + juice of 1 lemon or orange. Tastes surprisingly close to Pocari Sweat.

Pocari Sweat itself might be available at Asia Market Dublin — worth checking their drinks section or asking staff.

Other drinks to try

Based on your sweet-fruity-mild profile:

Yuzu tea (yujacha, 유자차) — Korean citrus marmalade-style tea. Sweet, fragrant, easy. Available at Asian stores in jars — just add hot or cold water. Very popular and widely stocked.
Ribena (no added sugar) — blackcurrant, sweet, smooth. Available everywhere in Ireland.
Vimto squash — grape/raspberry/blackcurrant blend. Available in Tesco Ireland.
Barley tea (bori-cha, 보리차) — roasted, nutty, mildly sweet. Caffeine-free. Very easy — just steep barley tea bags in cold water. Available at Asian stores.
MiWadi — Irish fruit squash brand, lots of flavours. Not fancy but hits the sweet-fruity-easy target.
Fruit tisanes / herbal teas — brands like Pukka, Twinings, or Barry's do fruit blends. Brew and chill for iced versions.
Coconut water — naturally sweet and mild. Available in most Irish supermarkets (Vita Coco, Tesco own brand).

If you're near Dublin, a trip to Asia Market on Drury Street is the single best move — you'll find maesil syrup, yuzu tea, barley tea, and possibly Pocari Sweat all in one go.