You need to ask the particular Credit Union but the general jist is that a loan would be paid off subject to the amount and age. The savings were often doubled but there ay be conditions here.
The 'common bond' is what allows you to join a particular Credit Union.
The common bond is a restriction on trade inherited from the US. That said it is in the law.
However, one CU in Dundalk - Black Raven (?) has as its common bond membership of a particular club which apparently all 4.4m of us are qualified to join.
Not surprising therefore that County Louth has more members than sheep.