They could get back the unused portion of their tax credits and a USC refund.
For the scenario you outline, assuming each person is an employee with typical tax credits (rather than self-employed where the credits are slightly different), the big earner with all credits would have €4950 annual credits (employee credit of €1650 and married couple credits of €3300). By the end of June, they would have been granted half of this amount, €2475. Assuming that their year to date income tax exceeds €2475 by the end of June (a safe assumption unless they have some massive tax breaks not outlined), they can get back their unused tax credits.
For the USC refund- I calculate that their USC deduction by the end of June is €6428 but their actual liability on an annual income of €100000 would be €4856, so a refund of €1572.