Modern usage of the diacritics in Welsh is as follows:
The circumflex is used solely to indicate that a vowel is long in a context in which it would normally be expected to be short, e.g.:
The diaeresis is used to separate vowels, as in English
The acute accent is used to indicate unexpected stress (i.e. not on the penultimate):
The grave accent is used to indicate that a vowel is short in a context in which it would normally be expected to be long:
Generally speaking, diacritics in Welsh cannot reasonably be omitted as they are used either to show unusual stress, or to differentiate between pairs of otherwise identical words with different pronounciations. As such they are equally necessary in upper- and lower-case forms.
The commonest diacritic is the circumflex, followed by the acute and diaeresis probably about equally. The grave is rare, but as more and more words are borrowed from English, and new compounds coined for technical terms, their use will undoubtedly increase.
To give a very rough indication, according to the headwords in our (unfinished) dictionary (which we estimate will contain about about 84,500 entries), the number of accented keywords (extrapolated to the expected finished size of the dictionary) will be roughly: