Do you have access to Oman Volume VII or Robertson, “Wellington Invades France”? I think the short answer to your question lies in page 53 of the former and page 120 of the latter. However, the longer answer would also require some further reading of preceding pages, involving the discussion of Wellington’s strategy: covering the siege of San Sebastian and the repulse of Soult’s attempts to relieve the place (in the short term) and also considerations related to the possible invasion of France (in the longer term) and the situation of the Anglo, Portuguese and Spanish forces.

Whilst a literal reading of the phrase “the loss there was unavoidable: that of Le Cor [several officers and 70 men], I must confess, was owing in some degree to the too forward gallantry of the Portuguese troops" may mean “that they were too bold and exposed themselves to superior enemy forces” as Digby suggests, it could perhaps have another, slightly more nuanced, meaning.

Firstly, as I understand it total losses of the Anglo Portuguese forces were in the order of 250 against 325 French. A not inconsiderable, but acceptable, loss in light of the difficulties of the terrain and weather and the strength of the French position (and their quite stubborn resistance). However, I think the main points are as follows:

1) the attack was meant as a feint only (although it is argued by Oman, and Robertson who takes his lead), that the feint was made much more credible by the impetuosity of the Portuguese attack
2) Le Cor’s Portuguese troops were initially successful in driving out Rémond’s French defenders at Zugarramurdi but were repulsed (with Madden) when following up their initial success and either fell back onto Barnes or were reinforced (I do not know which)
3) Oman and Robertson do not say if Dalhousie ordered, sanctioned the attack or merely permitted it to take place
4) despite the feint having the desired effect, Dalhousie’s description in your cited section from “dispatches” suggests that he is trying to distance himself from the attack somewhat due:
a) to the losses incurred unnecessarily and
b) the fact that the impetuosity of the attack was perhaps against Wellington’s orders, that the “attack” was to be merely a feint or ruse to stop Soult’s grander scheme to relieve San Sebastian and
c) perhaps also given his relatively poor handling of British troops sometime earlier.

I hope this helps your understanding, if you’d like me to read through the section in a bit more detail and offers some further thoughts I’d be happy to help.

Best wishes

Ant