Piping first. If you can't get all the piping in before the walk, you can install empty 4" PVC sleeves ("sealed" with unglued caps) that span the walk width, in the most likely areas where irrigation pipes would be run. Later, you can run pipes and/or wires through these sleeves.

We do hardscape first. Irrigation guys do better if they have fixed points to work to. It's why my preferred order of operations is actually hardscape -> trees & shrubs -> irrigation -> perennials, sod, mulch, cleanup. That way they know exactly where the heads need to be. If they have to guess, the odds aren't in our favor.

When the walkway is built, we put a 4" diameter pvc pipe under the walk any place that we think we, or the irrigation guys, will want to cross the walk. Cover the ends of the pipe, stick a flag at each end so they're easy to find again, good to go.

we hired a landscape company that did irrigation too. actually, they use a separate company for irrigation, but the landscape company is top dog.

what i have found is every time i hire two companies to do intertwined work, they always play the blame game about what work should have been done first or shouldnt have already been done or 'your guy messed up our stuff' and things like that. if its one person in charge of it all (and that person isnt you) you'll have fewer grey hairs. any do overs is on them.

My answer was predicated on the belief that the owner was doing the work himself or at least in direct supervision, and therefore in control of everything. If not, then I'd do as Rev. Gardens said.