I recently discovered that certain UK keyboards can actually be used to type Welsh characters. These instructions are assuming Windows 7, though they should be identical for Windows Vista and relatively similar for Windows XP and Windows 8. It should be possible to do something similar in Linux or OS X, but I couldn’t tell you how.
Firstly, you want to open up the control panel. If your control panel is set to display by category, you want to enter Clock, Language and Region, followed by Change keyboards or other input methods. If it is set to just display everything, you want to enter Region and Language, followed by Keyboards and Languages (on my computer, it’s the third tab in the window). Both methods result in you being in the same window, from which you click Change Keyboards.
In the window that opens, simply click Add, and then scroll down to Welsh. From there, expand out until you see the options “United Kingdom” and “United Kingdom Extended”. Select United Kingdom Extended, and then click OK. From there, you can choose whether to have Welsh or English as your default input language, and if memory serves, you should get a little “EN” or “CY” on the task bar to show which you have selected. If this doesn’t happen, go to the Language Bar tab in the same window you used to add the Welsh keyboard, and activate it there.
Once all that’s done, the rest is really easy. For a è, you press the ‘`’ key, followed by the vowel you want. For a é, you press Ctrl, Alt and the vowel you want (if your keyboard has Alt Gr, you may also use that for anything that uses Ctrl and Alt). For a ê, you use Ctrl, Alt and 6, followed by the vowel you want. Finally, for a ë, you use Ctrl, Alt and 2, followed by the vowel you want.
On a US keyboard, the keys required will probably change (it’ll probably be the single quote key instead of the 2 key for a ë, for instance, and whichever number in the US layout has ^ above it for ê).
Just thought that little tidbit might be useful.