the problem is if the money they get isn't as simple as "the current salary"

also the chance that they a) don't have a sentiment about meta, b) ex-colleges they have good relations too and c) now about some internal workings is close to nothing

a) a is bad no matter if it's a positive or negative

b) is very bad, it pretty much guarantees some degree of non objective actions. Like in positions like lobbyist it's never "just bussiness" it's always about positive connections reinforced by positive interactions not "big" enough to be bribes but also not nothing. Which might be related to how they got this job.

c) is a potential even bigger problem, if the sentiment is negative this insider knowledge could be used to harm meta, which in turn gives meta munition to sue and hinder regulatory actions and for a positive sentiment they might subtle change decisions because they know how they "happen to unluckily collide with how meta does things" and similar. But they really shouldn't do that.

Worse even if the person acts perfectly neutral meta can try to fight/delay legislation just by "claiming" this person did abuse insider knowledge.

Lastly how do you know that there isn't an unofficial deal that meta will pick them up again with a superb salary after they happened to do subtly meta friendly politics.

Like don't get me wrong, the person might be cable to act neutral and there might not be any under the table agreements. It's even quite likely. But it's a pretty bad idea anyway because it stinks of corruption no matter if it actually involves corruption.

And it's not like Irland has a problem with decisions biased in benefit for big US tech they have even been sued over by the EU....

They will if they want to work in the industry again.

There's no inherent reason they wouldn't retain favor for their former colleagues and friends from my viewpoint

I mean positive sentiment between people they worked with might literally have gotten them the job (as they most likely worked with people which have influence on this hiring decision during their time as lobbyist. And a huge part of lobby work is to create positive personal connections with people from the other side through whatever mean is possible which doesn't count as bribe. You could say positive personal sentiment is the WD40(lubricant) of smooth manipulation of political outcomes)

https://en.wikipedia.org/wiki/Revolving_door_(politics) => huge potential for conflicts of interest. That's why you should avoid this sort of thing in most cases.

I remember reading (here's the trigger for some people to stop reading) Paul Krugman's writing about Greek politicians and the Brussels set: if the Greek leaders just follow what Brussels want, they can guarantee their spot in some cushy position afterwards, and what Brussels set (lobbied by German banks) wanted was austerity for Greeks in order to repay the loans the German banks gave out freely, knowing that because the country was in the Eurozone, the ECB will bail them out.

Meanwhile the socialist Syriza party were obviously former activists who were going to fight for the Greek people, and Brussels knew they won't be persuaded to follow the script...

Result is that Greece's economy has never been doing better, their budget produces a surplus and they can reimburse their debt.