after conquering a battlefield can i pay 1 to put irelia trigger conquer ability on the chain even when the legend is already ready and in reaction use a en garde to choose unit to exhaust legend and ready unit then ready legend?

Yes — the line works. One clarification first: En Garde itself only gives +1 might; the "exhaust legend / ready unit" part is Blade Dancer's other ability, not En Garde's effect.

Per FAQ #12399 you can finalize Blade Dancer's conquer trigger ("When you conquer, you may pay [1] to ready me") and pay the 1 even though the legend is already ready — the cost is just energy, not exhausting anything, and readying an already-ready object is a no-op (CR 416.1.c). Then, with that trigger still on the chain, you play En Garde (it has Reaction) targeting a friendly unit. Choosing that unit triggers Blade Dancer's first ability, "When you choose a friendly unit, you may exhaust me and pay [A] to ready it," which you finalize by exhausting the (still-ready) legend and paying [A].

Chain resolves last-in-first-out: Blade Dancer's choose trigger readies the chosen unit, En Garde gives it +1 might, then the conquer trigger resolves and readies the legend — which is now exhausted, so it's no longer a no-op (CR 416.1.c). Net: legend ends ready, unit readied, cost is 1 + 1 + [A]. Note the legend must be ready when you finalize the Blade Dancer trigger; if it were already exhausted you couldn't pay that exhaust cost (CR 415.4).