A lot — orientation and shading are among the biggest determinants of how well your system performs, and getting them wrong is a frequent source of regret. South-facing is the ideal, delivering the highest output. East/west splits are common and perfectly workable (and can spread generation across morning and evening, which suits some households' usage), but north-facing is genuinely poor — especially for the six darker months — and panels placed there to suit an easy install rather than good production are a real complaint people raise.
Shading is the other big factor, and it punishes you more than you'd expect. Because panels on a string are wired in series, even minor shade on a single panel — from a chimney, a tree, or a neighbour's roof — can drag down the output of the whole string. The data shows the impact of partial shading is often far larger than the size of the shadow suggests. The standard Irish remedy is optimisers on the affected panels, which isolate a shaded panel so it doesn't throttle its neighbours, and these are well worth it where shade is unavoidable.
The practical advice: prioritise your best-producing roof faces, be wary of an installer steering panels onto a worse orientation for installation convenience, and factor in evening sun if your usage is heavily evening-weighted (a west face can be valuable for an evening-occupied home even though it produces less total than south). If any panels will be shaded at some point in the day, budget for optimisers on those. Designing for orientation and shade up front beats discovering the panels mostly generate while you're out at work.