This is no longer to do with the original subject (sorted?), but often my aboveground farming is made by channelling out the ground-level tiles, either into pre-dug Z-1 areas[1] or removing whatever upward ramps remain at the end[2]. Even with quite heavy 'boulder field' conditions at ground-level will (assuming it is a soild soil-layer on Z-1) produce an uninterrupted farmable soil surface on Z-1. And non-pathable-to, what's more (except for routes through the fortress entrance, or unremoved edge-ramps, and of course by fliers, but there's ways round that as well), while still being an official above-ground farm.
And I don't think I've had to irrigate such a setup recently. If ever, as I'm not sure they had to be even when sub-surface (soil and rock, for some releases, just rock for others with soil being Ok) needed irrigation to make it viable subsurface farmland.
Anyway, currently if it's subterranean farming, then on soil (and near-equivalents) that's never been sky-exposed it's not... unless I've missed something, so please correct me... necessary to irrigate at all, these days. Right? I mean, I've played .34 versions and set up such fields and I can't remember having to irrigate. But, as the Scot's might ask: "A meringue?"
[1] Being careful to use patterns of channel digging that avoids cave-in conditions.
[2] Some will, but ramps disappear if their sole remaining 'wall' is removed so any "the last block among its neighbours to be channelled from above" will become a ramp.