Is that the actual problem? Or is the problem that the tribal areas have too little population?
Ireland at the start of the game has 124 pops in all its territories, including the uncolonized ones. That's only slightly more than three time as many as in the territory of Syracuse (the city itself that is). was the population really that low? I know the population density was lower than further south, but was it really that low?
You're not going to find reliable population numbers for Ireland in the fourth century BCE. Maybe some estimate based on theoretical carrying capacity, but it'll be shaky as all hell.
Maddison has an estimate for the British Isles
in toto at 800k in 1 CE, with numbers of 8m and 2m for (within their modern borders) Italy and Greece at that time, respectively. The table
on Wikipedia (I think from Frier?) roughly agrees with this (it's defining "Greece" and "Italy" differently), though it does not address Britannia. The first figure Maddison gives for Ireland by itself is 800k... but that's in 1500 CE.
800k for the isles would imply a population density of 2.5 persons per square kilometre, which
would be very low compared to the densities listed for the other regions of the barbarian west in the Wikipedia table. At 2.5 p/km2 Ireland would have had a population of 200k~ ish, about 1/40th of the combined population of the in-game regions of Cisalpine Gaul, Italia and Magna Graeca. Something more in-line with the densities for other regions would be 5 p/km2, or 400k total- 1/20th of Italy.
Though this is of course at the turn of the millennium, and the game starts three hundred years earlier.
In game, I find a total population for Ireland, including uncolonised regions, of 124 pops. For Italy, meaning the three regions of Cisalpine Gaul, Italia and Magna Graeca, including the uncolonised portion of Corsica, I find 3406. This is a ratio of 1:27... which falls between our extrapolation from Maddison and that from Frier. Closer to the latter (higher) value than the former, in fact.
You could of course make an argument for densities closer to that of Gaul (so a ratio with Italy nearer 1:10)... but Gaul was fairly rich, agriculturally.