Marlin Robe is crafted with Tailoring in Mabinogi (robe). This page lists everything needed to make it, checked against the game as it exists today. The interactive planner lives in the Tailoring Companion.
How to get the sewing pattern
The sewing pattern is rank 5 and survives 30 uses; every crafting attempt spends one use.
No working source exists today. Player trades and the auction house are the only path.
Materials
Expect about 5 attempts per finished item (21% average progress per attempt). Per-attempt materials are consumed on every attempt; finishing materials once per item.
Each attempt uses
2× Common Fabric — buy from NPC shops
2× Fine Leather — fragmentation of Fine Leather Strap (or weaving byproduct)
1× Flying Fish Fin — Fail the Flying Fish Crepe recipe by cooking without egg or potato.
Finishing (once per item)
1× Finest Finishing Thread — Buy directly from any general goods NPC shop.
1× Striped Marlin Head — Likely obtained through deep sea fishing, same source as the Striped Marlin fish.