The tilta is a 'fada' in Irish (Fada meaning long) and it would be the equivalent to the 'Accent' in French. The effect is to extend the enunciation of the 'a' on Bodhrán so it is pronounced as equivalent to 'Bow-Rawn' as opposed to 'Bow-Ran'. The Irish word Bodhrán comes from the Irish word Bodhar (Pronounced 'Bower') meaning Deaf. A literal translation of Bodhrán therefore could be 'Deafener' and in some hands I have heard it played to its literal meaning.

To achieve the 'fada' on the 'a' hold down the Ctrl and Alt keys when typing the 'a'. It seem to only work with vowels on the keyboard which stands to reason come to think of it as these are the only sounds that can be affected by such an accent.