Originally posted as a Github wiki
In order to permanently remap the Shift+3 combination in a default Irish keyboard in Ubuntu, follow these steps.
Firstly, navigate to the folder where the default keymaps are stored. This is
/usr/share/X11/xkb/symbols
Edit the Irish keyboard layout using nano or your text editor of choice:
sudo nano ie
The default combination for Shift + 3 is to display the Sterling currency symbol.
key <TLDE> { [ grave, notsign, brokenbar, >
key <AE01> { [ 1, exclam, exclamdown, on>
key <AE02> { [ 2, quotedbl, trademark, tw>
key <AE03> { [ 3, sterling, copyright, th>
Remap it to numbersign (not ‘hashtag’!)
key <TLDE> { [ grave, notsign, brokenbar, >
key <AE01> { [ 1, exclam, exclamdown, on>
key <AE02> { [ 2, quotedbl, trademark, tw>
key <AE03> { [ 3, numbersign, copyright, th>
Save changes.
Reload the keymap to verify that the change has been applied correctly:
setxkbmap -layout ie
Now you can use the Irish keyboard layout in Ubuntu while retaining quick access to create hashtags for Twitter like in the US layout!
(These changes will persist through a reboot).