Unlike verbs, adjectives are a bit more difficult to handle. Like English, they change in the comparative and superlative forms (e.g. big, bigger and biggest). However, they also change slightly depending on the case (tuiseal) being used.
Usually the form to use is easy figure out. But sometimes there can be doubts, not helped by the fact that ordinary printed dictionaries do not always provide the various forms. So, it is good know that there are some useful computer-based resources available.
This particular adjective (meaning big) is simple and well-known enough (though irregular), but what about others that are a little less well-known, e.g. nua (meaning new; níos nua? nuaí? nuaidhí?). Sometimes one is not completely sure.
WinGléacht is the best resource for this. Just insert mór, for example, into the search box, and press enter. This particular word has several meanings, so you need to find the one that is the adjective (indicated as aid), and right-click. All forms of the adjective are given.
Unlike in English, adjectives in Irish change depending on the case. It is not our goal to explain the details here, but just to say that the various forms can also be found when the selection is made in WinGléacht above. Really, what you might be looking for is the genitive form (ginideach).
Note that the various forms of the adjective (for the cases only) are also provided in An Foclóir Beag.
WinGléacht is a commercial product, i.e. not free. It is discussed elsewhere on this site, under the column Translation/WinGléacht.