Just to reiterate my own position and situation - I am a British citizen, raised in Ireland and currently based in Singapore. Having lived in Brentwood for six years, and with every intention of returning, I am registered overseas as a voter in the parliamentary constituency of Brentwood and Ongar. This gives me the right to vote in General Elections and referenda, but not local elections in Brentwood.
Despite some of the rhetoric used, I've come to the conclusion that voting is not a human right - at least not in the sense that the right to freedom of religion or the right to be fed and sheltered is. I base this on a number of reasons, but the most obvious one is that a French citizen living in the UK or Ireland does not get the vote. Instead of which, the right to vote is a privilege of the citizenship you hold - and further, other restrictions can apply to that right (eg an Irish citizen must actually be resident in the UK to exercise the right of an Irish citizen to vote in a UK election).
It is however clear to me that any restrictions on voting rights must be clearly objective in a proper democracy (the European Court ruling on prisoners voting rights implied it was okay to stop them voting if their sentence was more than four years). British rules on emigrant voting rights are clear - the overseas vote is only allowed for 15 years.
The second point that needs to be raised is that under the electoral systems used in much of the former British Empire (which includes the US and Ireland) you typically vote for the representative in a specific area. When I cast my vote in the next UK election, the only thing I'm actually voting on is whether or not I want Eric Pickles or one of his opponents to represent Brentwood and Ongar in the House of Commons - I am not voting on whether I'd prefer David Cameron or Ed Miliband to be Prime Minister (though the position of the candidates themselves on that issue might influence my voting decision).
The final point is that the issue of overseas citizens voting gets even more complicated when one considers citizenship by descent - despite the failings of successive Fianna Fail governments, I doubt Ireland would have been better off if in the 1980s the romanticised view of Ireland amongst Irish Americans had ended up leading to second and third generation Irish American votes effectively imposing Sinn Fein on the voters who would have had to live with the day to day consequences of that decision.
Noreen Bowden has made the point that the young Irish people who have had to emigrate as a result of the Irish economy imploding don't get a chance to determine the government who might be able to fix the mess so that they could come back in a few years. She's right - but I do think there needs to be a balance between those who have the intention to return and those who ultimately decide to remain abroad for the remainder of their lives. Ultimately, in my view, there needs to be a balance between those who may or will be affected in their day to day lives by the decisions made at election at time, and those for whom the biggest impact may be reduced consular services in Timbuktu. I do think tax liability plays a part here - the US taxes American citizens abroad, Ireland and the UK do not.
So how do you tell when a leaver has the intent to return? Well, that's actually in itself horribly subjective - and open to change. So you can't. Instead, you end up having to approximate - suggesting that there's a fixed period of time within which you retain the right to vote, but after that it's accepted you don't. In other words, I think the UK has it more or less right - British citizens retain the right for 15 years after departure. You can argue about the specific length (why not 10 or 20) but in general, I think the balance is right.
That's not to say there aren't other opportunities to engage emigrants in the political process. It has always been my view that having a two house legislature where both are elected on a proportional basis is a bad idea - the "Lower" house, which tends to be more representative of how votes are cast, should always have the precedence - and if both houses are fully and directly elected it's then unclear which has the mandate and precedence. The role of the upper house needs to be to act as a restraint on politicians simply passing idiotic populist legislation and to provide alternative voices within the system. My advice therefore to my Irish friends is that they should campaign for the right to vote to be retained for a fixed number of years, and that as part of the reform of the Seanad, they could also have a specific number of Senators to represent the views of Irish citizens abroad.
What do you think?
 Yes, I know there's a few big ifs in there, but I don't think by themselves they are unreasonable.
 Ironically, despite the tendency of Singaporeans to label those who do move abroad as quitters, there's actually a concerted effort to get those citizens to vote as overseas voters in the elections. And that's despite the evidence that overseas voters are much more likely to vote for Opposition candidates.
 The issue of tax liability is not of course the determining factor in itself. Plenty of emigrants still have investments back in the homeland, but that's a factor in how they deploy their investments, it's not automatic.
 Another artifact of the British legacy?