Not totally sure since I don't have whatever mod adds ironwood trees, but two possible things to try:
- The leaf shearing addon for the gatherer, it makes it collect leaves rather than breaking them but if my dusty old memory serves i've used it before to collect cherries from Forestry cherry trees.
- If you can collect the berries with right click (ignore the next bit if this isn't the case), use the Plant Interactor rather than the Gatherer.
Alternately, you can keep your setup as-is and plonk down some OpenBlocks vacuum hoppers at floor level and they'll suck in any ironberries that drop.